515 lines
14 KiB
ReStructuredText
515 lines
14 KiB
ReStructuredText
=========================================
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How to get printk format specifiers right
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=========================================
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:Author: Randy Dunlap <rdunlap@infradead.org>
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:Author: Andrew Murray <amurray@mpc-data.co.uk>
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Integer types
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=============
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::
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If variable is of Type, use printk format specifier:
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------------------------------------------------------------
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char %hhd or %hhx
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unsigned char %hhu or %hhx
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short int %hd or %hx
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unsigned short int %hu or %hx
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int %d or %x
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unsigned int %u or %x
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long %ld or %lx
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unsigned long %lu or %lx
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long long %lld or %llx
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unsigned long long %llu or %llx
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size_t %zu or %zx
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ssize_t %zd or %zx
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s8 %hhd or %hhx
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u8 %hhu or %hhx
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s16 %hd or %hx
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u16 %hu or %hx
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s32 %d or %x
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u32 %u or %x
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s64 %lld or %llx
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u64 %llu or %llx
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If <type> is dependent on a config option for its size (e.g., sector_t,
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blkcnt_t) or is architecture-dependent for its size (e.g., tcflag_t), use a
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format specifier of its largest possible type and explicitly cast to it.
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Example::
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printk("test: sector number/total blocks: %llu/%llu\n",
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(unsigned long long)sector, (unsigned long long)blockcount);
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Reminder: sizeof() returns type size_t.
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The kernel's printf does not support %n. Floating point formats (%e, %f,
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%g, %a) are also not recognized, for obvious reasons. Use of any
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unsupported specifier or length qualifier results in a WARN and early
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return from vsnprintf().
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Pointer types
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=============
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A raw pointer value may be printed with %p which will hash the address
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before printing. The kernel also supports extended specifiers for printing
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pointers of different types.
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Some of the extended specifiers print the data on the given address instead
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of printing the address itself. In this case, the following error messages
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might be printed instead of the unreachable information::
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(null) data on plain NULL address
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(efault) data on invalid address
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(einval) invalid data on a valid address
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Plain Pointers
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--------------
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::
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%p abcdef12 or 00000000abcdef12
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Pointers printed without a specifier extension (i.e unadorned %p) are
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hashed to prevent leaking information about the kernel memory layout. This
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has the added benefit of providing a unique identifier. On 64-bit machines
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the first 32 bits are zeroed. The kernel will print ``(ptrval)`` until it
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gathers enough entropy. If you *really* want the address see %px below.
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Symbols/Function Pointers
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-------------------------
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::
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%pS versatile_init+0x0/0x110
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%ps versatile_init
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%pF versatile_init+0x0/0x110
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%pf versatile_init
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%pSR versatile_init+0x9/0x110
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(with __builtin_extract_return_addr() translation)
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%pB prev_fn_of_versatile_init+0x88/0x88
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The ``S`` and ``s`` specifiers are used for printing a pointer in symbolic
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format. They result in the symbol name with (S) or without (s)
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offsets. If KALLSYMS are disabled then the symbol address is printed instead.
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Note, that the ``F`` and ``f`` specifiers are identical to ``S`` (``s``)
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and thus deprecated. We have ``F`` and ``f`` because on ia64, ppc64 and
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parisc64 function pointers are indirect and, in fact, are function
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descriptors, which require additional dereferencing before we can lookup
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the symbol. As of now, ``S`` and ``s`` perform dereferencing on those
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platforms (when needed), so ``F`` and ``f`` exist for compatibility
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reasons only.
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The ``B`` specifier results in the symbol name with offsets and should be
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used when printing stack backtraces. The specifier takes into
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consideration the effect of compiler optimisations which may occur
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when tail-calls are used and marked with the noreturn GCC attribute.
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Kernel Pointers
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---------------
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::
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%pK 01234567 or 0123456789abcdef
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For printing kernel pointers which should be hidden from unprivileged
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users. The behaviour of %pK depends on the kptr_restrict sysctl - see
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Documentation/sysctl/kernel.rst for more details.
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Unmodified Addresses
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--------------------
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::
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%px 01234567 or 0123456789abcdef
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For printing pointers when you *really* want to print the address. Please
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consider whether or not you are leaking sensitive information about the
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kernel memory layout before printing pointers with %px. %px is functionally
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equivalent to %lx (or %lu). %px is preferred because it is more uniquely
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grep'able. If in the future we need to modify the way the kernel handles
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printing pointers we will be better equipped to find the call sites.
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Struct Resources
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----------------
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::
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%pr [mem 0x60000000-0x6fffffff flags 0x2200] or
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[mem 0x0000000060000000-0x000000006fffffff flags 0x2200]
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%pR [mem 0x60000000-0x6fffffff pref] or
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[mem 0x0000000060000000-0x000000006fffffff pref]
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For printing struct resources. The ``R`` and ``r`` specifiers result in a
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printed resource with (R) or without (r) a decoded flags member.
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Passed by reference.
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Physical address types phys_addr_t
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----------------------------------
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::
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%pa[p] 0x01234567 or 0x0123456789abcdef
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For printing a phys_addr_t type (and its derivatives, such as
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resource_size_t) which can vary based on build options, regardless of the
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width of the CPU data path.
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Passed by reference.
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DMA address types dma_addr_t
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----------------------------
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::
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%pad 0x01234567 or 0x0123456789abcdef
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For printing a dma_addr_t type which can vary based on build options,
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regardless of the width of the CPU data path.
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Passed by reference.
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Raw buffer as an escaped string
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-------------------------------
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::
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%*pE[achnops]
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For printing raw buffer as an escaped string. For the following buffer::
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1b 62 20 5c 43 07 22 90 0d 5d
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A few examples show how the conversion would be done (excluding surrounding
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quotes)::
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%*pE "\eb \C\a"\220\r]"
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%*pEhp "\x1bb \C\x07"\x90\x0d]"
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%*pEa "\e\142\040\\\103\a\042\220\r\135"
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The conversion rules are applied according to an optional combination
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of flags (see :c:func:`string_escape_mem` kernel documentation for the
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details):
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- a - ESCAPE_ANY
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- c - ESCAPE_SPECIAL
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- h - ESCAPE_HEX
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- n - ESCAPE_NULL
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- o - ESCAPE_OCTAL
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- p - ESCAPE_NP
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- s - ESCAPE_SPACE
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By default ESCAPE_ANY_NP is used.
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ESCAPE_ANY_NP is the sane choice for many cases, in particularly for
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printing SSIDs.
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If field width is omitted then 1 byte only will be escaped.
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Raw buffer as a hex string
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--------------------------
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::
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%*ph 00 01 02 ... 3f
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%*phC 00:01:02: ... :3f
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%*phD 00-01-02- ... -3f
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%*phN 000102 ... 3f
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For printing small buffers (up to 64 bytes long) as a hex string with a
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certain separator. For larger buffers consider using
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:c:func:`print_hex_dump`.
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MAC/FDDI addresses
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------------------
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::
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%pM 00:01:02:03:04:05
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%pMR 05:04:03:02:01:00
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%pMF 00-01-02-03-04-05
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%pm 000102030405
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%pmR 050403020100
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For printing 6-byte MAC/FDDI addresses in hex notation. The ``M`` and ``m``
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specifiers result in a printed address with (M) or without (m) byte
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separators. The default byte separator is the colon (:).
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Where FDDI addresses are concerned the ``F`` specifier can be used after
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the ``M`` specifier to use dash (-) separators instead of the default
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separator.
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For Bluetooth addresses the ``R`` specifier shall be used after the ``M``
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specifier to use reversed byte order suitable for visual interpretation
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of Bluetooth addresses which are in the little endian order.
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Passed by reference.
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IPv4 addresses
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--------------
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::
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%pI4 1.2.3.4
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%pi4 001.002.003.004
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%p[Ii]4[hnbl]
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For printing IPv4 dot-separated decimal addresses. The ``I4`` and ``i4``
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specifiers result in a printed address with (i4) or without (I4) leading
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zeros.
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The additional ``h``, ``n``, ``b``, and ``l`` specifiers are used to specify
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host, network, big or little endian order addresses respectively. Where
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no specifier is provided the default network/big endian order is used.
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Passed by reference.
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IPv6 addresses
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--------------
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::
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%pI6 0001:0002:0003:0004:0005:0006:0007:0008
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%pi6 00010002000300040005000600070008
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%pI6c 1:2:3:4:5:6:7:8
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For printing IPv6 network-order 16-bit hex addresses. The ``I6`` and ``i6``
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specifiers result in a printed address with (I6) or without (i6)
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colon-separators. Leading zeros are always used.
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The additional ``c`` specifier can be used with the ``I`` specifier to
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print a compressed IPv6 address as described by
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http://tools.ietf.org/html/rfc5952
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Passed by reference.
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IPv4/IPv6 addresses (generic, with port, flowinfo, scope)
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---------------------------------------------------------
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::
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%pIS 1.2.3.4 or 0001:0002:0003:0004:0005:0006:0007:0008
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%piS 001.002.003.004 or 00010002000300040005000600070008
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%pISc 1.2.3.4 or 1:2:3:4:5:6:7:8
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%pISpc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345
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%p[Ii]S[pfschnbl]
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For printing an IP address without the need to distinguish whether it's of
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type AF_INET or AF_INET6. A pointer to a valid struct sockaddr,
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specified through ``IS`` or ``iS``, can be passed to this format specifier.
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The additional ``p``, ``f``, and ``s`` specifiers are used to specify port
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(IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a ``:`` prefix,
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flowinfo a ``/`` and scope a ``%``, each followed by the actual value.
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In case of an IPv6 address the compressed IPv6 address as described by
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http://tools.ietf.org/html/rfc5952 is being used if the additional
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specifier ``c`` is given. The IPv6 address is surrounded by ``[``, ``]`` in
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case of additional specifiers ``p``, ``f`` or ``s`` as suggested by
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https://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-07
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In case of IPv4 addresses, the additional ``h``, ``n``, ``b``, and ``l``
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specifiers can be used as well and are ignored in case of an IPv6
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address.
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Passed by reference.
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Further examples::
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%pISfc 1.2.3.4 or [1:2:3:4:5:6:7:8]/123456789
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%pISsc 1.2.3.4 or [1:2:3:4:5:6:7:8]%1234567890
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%pISpfc 1.2.3.4:12345 or [1:2:3:4:5:6:7:8]:12345/123456789
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UUID/GUID addresses
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-------------------
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::
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%pUb 00010203-0405-0607-0809-0a0b0c0d0e0f
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%pUB 00010203-0405-0607-0809-0A0B0C0D0E0F
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%pUl 03020100-0504-0706-0809-0a0b0c0e0e0f
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%pUL 03020100-0504-0706-0809-0A0B0C0E0E0F
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For printing 16-byte UUID/GUIDs addresses. The additional ``l``, ``L``,
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``b`` and ``B`` specifiers are used to specify a little endian order in
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lower (l) or upper case (L) hex notation - and big endian order in lower (b)
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or upper case (B) hex notation.
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Where no additional specifiers are used the default big endian
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order with lower case hex notation will be printed.
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Passed by reference.
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dentry names
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------------
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::
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%pd{,2,3,4}
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%pD{,2,3,4}
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For printing dentry name; if we race with :c:func:`d_move`, the name might
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be a mix of old and new ones, but it won't oops. %pd dentry is a safer
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equivalent of %s dentry->d_name.name we used to use, %pd<n> prints ``n``
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last components. %pD does the same thing for struct file.
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Passed by reference.
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block_device names
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------------------
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::
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%pg sda, sda1 or loop0p1
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For printing name of block_device pointers.
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struct va_format
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----------------
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::
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%pV
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For printing struct va_format structures. These contain a format string
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and va_list as follows::
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struct va_format {
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const char *fmt;
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va_list *va;
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};
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Implements a "recursive vsnprintf".
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Do not use this feature without some mechanism to verify the
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correctness of the format string and va_list arguments.
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Passed by reference.
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Device tree nodes
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-----------------
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::
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%pOF[fnpPcCF]
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For printing device tree node structures. Default behaviour is
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equivalent to %pOFf.
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- f - device node full_name
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- n - device node name
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- p - device node phandle
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- P - device node path spec (name + @unit)
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- F - device node flags
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- c - major compatible string
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- C - full compatible string
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The separator when using multiple arguments is ':'
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Examples::
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%pOF /foo/bar@0 - Node full name
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%pOFf /foo/bar@0 - Same as above
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%pOFfp /foo/bar@0:10 - Node full name + phandle
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%pOFfcF /foo/bar@0:foo,device:--P- - Node full name +
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major compatible string +
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node flags
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D - dynamic
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d - detached
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P - Populated
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B - Populated bus
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Passed by reference.
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Time and date (struct rtc_time)
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-------------------------------
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::
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%ptR YYYY-mm-ddTHH:MM:SS
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%ptRd YYYY-mm-dd
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%ptRt HH:MM:SS
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%ptR[dt][r]
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For printing date and time as represented by struct rtc_time structure in
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human readable format.
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By default year will be incremented by 1900 and month by 1. Use %ptRr (raw)
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to suppress this behaviour.
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Passed by reference.
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struct clk
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----------
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::
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%pC pll1
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%pCn pll1
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For printing struct clk structures. %pC and %pCn print the name of the clock
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(Common Clock Framework) or a unique 32-bit ID (legacy clock framework).
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Passed by reference.
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bitmap and its derivatives such as cpumask and nodemask
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-------------------------------------------------------
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::
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%*pb 0779
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%*pbl 0,3-6,8-10
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For printing bitmap and its derivatives such as cpumask and nodemask,
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%*pb outputs the bitmap with field width as the number of bits and %*pbl
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output the bitmap as range list with field width as the number of bits.
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Passed by reference.
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Flags bitfields such as page flags, gfp_flags
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---------------------------------------------
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::
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%pGp referenced|uptodate|lru|active|private
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%pGg GFP_USER|GFP_DMA32|GFP_NOWARN
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%pGv read|exec|mayread|maywrite|mayexec|denywrite
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For printing flags bitfields as a collection of symbolic constants that
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would construct the value. The type of flags is given by the third
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character. Currently supported are [p]age flags, [v]ma_flags (both
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expect ``unsigned long *``) and [g]fp_flags (expects ``gfp_t *``). The flag
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names and print order depends on the particular type.
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Note that this format should not be used directly in the
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:c:func:`TP_printk()` part of a tracepoint. Instead, use the show_*_flags()
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functions from <trace/events/mmflags.h>.
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Passed by reference.
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Network device features
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-----------------------
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::
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%pNF 0x000000000000c000
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For printing netdev_features_t.
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Passed by reference.
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Thanks
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======
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If you add other %p extensions, please extend <lib/test_printf.c> with
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one or more test cases, if at all feasible.
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Thank you for your cooperation and attention.
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