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[ELF] Add -z separate-loadable-segments to complement separate-code and noseparate-code 

D64906 allows PT_LOAD to have overlapping p_offset ranges. In the
default R RX RW RW layout + -z noseparate-code case, we do not tail pad
segments when transiting to another segment. This can save at most
3*maxPageSize bytes.

a) Before D64906, we tail pad R, RX and the first RW.
b) With -z separate-code, we tail pad R and RX, but not the first RW (RELRO).

In some cases, b) saves one file page. In some cases, b) wastes one
virtual memory page. The waste is a concern on Fuchsia. Because it uses
compressed binaries, it doesn't benefit from the saved file page.

This patch adds -z separate-loadable-segments to restore the behavior before
D64906. It can affect section addresses and can thus be used as a
debugging mechanism (see PR43214 and ld.so partition bug in
crbug.com/998712).

Reviewed By: jakehehrlich, ruiu

Differential Revision: https://reviews.llvm.org/D67481

llvm-svn: 372807
This commit is contained in:
Fangrui Song 2019-09-25 03:39:31 +00:00
parent 5e359a9a0e
commit 0264950697
6 changed files with 100 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
lld/ELF/Config.h
View File

@ -61,6 +61,9 @@ enum class Target2Policy { Abs, Rel, GotRel };
// For tracking ARM Float Argument PCS
enum class ARMVFPArgKind { Default, Base, VFP, ToolChain };
// For -z noseparate-code, -z separate-code and -z separate-loadable-segments.
enum class SeparateSegmentKind { None, Code, Loadable };
struct SymbolVersion {
llvm::StringRef name;
bool isExternCpp;
@ -209,7 +212,6 @@ struct Configuration {
bool zOrigin;
bool zRelro;
bool zRodynamic;
bool zSeparateCode;
bool zText;
bool zRetpolineplt;
bool zWxneeded;
@ -222,6 +224,7 @@ struct Configuration {
Target2Policy target2;
ARMVFPArgKind armVFPArgs = ARMVFPArgKind::Default;
BuildIdKind buildId = BuildIdKind::None;
SeparateSegmentKind zSeparate;
ELFKind ekind = ELFNoneKind;
uint16_t emachine = llvm::ELF::EM_NONE;
llvm::Optional<uint64_t> imageBase;

26
lld/ELF/Driver.cpp
View File

@ -380,16 +380,30 @@ static bool getZFlag(opt::InputArgList &args, StringRef k1, StringRef k2,
return Default;
}
static SeparateSegmentKind getZSeparate(opt::InputArgList &args) {
for (auto *arg : args.filtered_reverse(OPT_z)) {
StringRef v = arg->getValue();
if (v == "noseparate-code")
return SeparateSegmentKind::None;
if (v == "separate-code")
return SeparateSegmentKind::Code;
if (v == "separate-loadable-segments")
return SeparateSegmentKind::Loadable;
}
return SeparateSegmentKind::None;
}
static bool isKnownZFlag(StringRef s) {
return s == "combreloc" || s == "copyreloc" || s == "defs" ||
s == "execstack" || s == "global" || s == "hazardplt" ||
s == "ifunc-noplt" || s == "initfirst" || s == "interpose" ||
s == "keep-text-section-prefix" || s == "lazy" || s == "muldefs" ||
s == "separate-code" || s == "nocombreloc" || s == "nocopyreloc" ||
s == "nodefaultlib" || s == "nodelete" || s == "nodlopen" ||
s == "noexecstack" || s == "nokeep-text-section-prefix" ||
s == "norelro" || s == "noseparate-code" || s == "notext" ||
s == "now" || s == "origin" || s == "relro" || s == "retpolineplt" ||
s == "separate-code" || s == "separate-loadable-segments" ||
s == "nocombreloc" || s == "nocopyreloc" || s == "nodefaultlib" ||
s == "nodelete" || s == "nodlopen" || s == "noexecstack" ||
s == "nokeep-text-section-prefix" || s == "norelro" ||
s == "noseparate-code" || s == "notext" || s == "now" ||
s == "origin" || s == "relro" || s == "retpolineplt" ||
s == "rodynamic" || s == "text" || s == "undefs" || s == "wxneeded" ||
s.startswith("common-page-size=") || s.startswith("max-page-size=") ||
s.startswith("stack-size=");
@ -950,7 +964,7 @@ static void readConfigs(opt::InputArgList &args) {
config->zRelro = getZFlag(args, "relro", "norelro", true);
config->zRetpolineplt = hasZOption(args, "retpolineplt");
config->zRodynamic = hasZOption(args, "rodynamic");
config->zSeparateCode = getZFlag(args, "separate-code", "noseparate-code", false);
config->zSeparate = getZSeparate(args);
config->zStackSize = args::getZOptionValue(args, OPT_z, "stack-size", 0);
config->zText = getZFlag(args, "text", "notext", true);
config->zWxneeded = hasZOption(args, "wxneeded");

12
lld/ELF/Writer.cpp
View File

@ -589,7 +589,8 @@ template <class ELFT> void Writer<ELFT>::run() {
return;
if (!config->oFormatBinary) {
writeTrapInstr();
if (config->zSeparate != SeparateSegmentKind::None)
writeTrapInstr();
writeHeader();
writeSections();
} else {
@ -2233,7 +2234,8 @@ template <class ELFT> void Writer<ELFT>::fixSectionAlignments() {
// maximum page size boundary so that we can find the ELF header at the
// start. We cannot benefit from overlapping p_offset ranges with the
// previous segment anyway.
if ((config->zSeparateCode && prev &&
if (config->zSeparate == SeparateSegmentKind::Loadable ||
(config->zSeparate == SeparateSegmentKind::Code && prev &&
(prev->p_flags & PF_X) != (p->p_flags & PF_X)) ||
cmd->type == SHT_LLVM_PART_EHDR)
cmd->addrExpr = [] {
@ -2342,7 +2344,8 @@ template <class ELFT> void Writer<ELFT>::assignFileOffsets() {
// If this is a last section of the last executable segment and that
// segment is the last loadable segment, align the offset of the
// following section to avoid loading non-segments parts of the file.
if (config->zSeparateCode && lastRX && lastRX->lastSec == sec)
if (config->zSeparate != SeparateSegmentKind::None && lastRX &&
lastRX->lastSec == sec)
off = alignTo(off, config->commonPageSize);
}
@ -2614,9 +2617,6 @@ static void fillTrap(uint8_t *i, uint8_t *end) {
// We'll leave other pages in segments as-is because the rest will be
// overwritten by output sections.
template <class ELFT> void Writer<ELFT>::writeTrapInstr() {
if (!config->zSeparateCode)
return;
for (Partition &part : partitions) {
// Fill the last page.
for (PhdrEntry *p : part.phdrs)

33
lld/docs/ld.lld.1
View File

@ -591,18 +591,21 @@ Force load of all members in a static library.
Use wrapper functions for symbol.
.It Fl z Ar option
Linker option extensions.
.Bl -tag -width indent
.Bl -tag -width indent -compact
.Pp
.It Cm execstack
Make the main stack executable.
Stack permissions are recorded in the
.Dv PT_GNU_STACK
segment.
.Pp
.It Cm global
Sets the
.Dv DF_1_GLOBAL flag in the
.Dv DYNAMIC
section.
Different loaders can decide how to handle this flag on their own.
.Pp
.It Cm ifunc-noplt
Do not emit PLT entries for ifunc symbols.
Instead, emit text relocations referencing the resolver.
@ -611,64 +614,78 @@ environments where text relocations do not have the usual drawbacks.
This option must be combined with the
.Fl z Li notext
option.
.Pp
.It Cm initfirst
Sets the
.Dv DF_1_INITFIRST
flag to indicate the module should be initialized first.
.Pp
.It Cm interpose
Set the
.Dv DF_1_INTERPOSE
flag to indicate to the runtime linker that the object is an interposer.
During symbol resolution interposers are searched after the application
but before other dependencies.
.Pp
.It Cm muldefs
Do not error if a symbol is defined multiple times.
The first definition will be used.
This is a synonym for
.Fl -allow-multiple-definition.
.Pp
.It Cm nocombreloc
Disable combining and sorting multiple relocation sections.
.Pp
.It Cm nocopyreloc
Disable the creation of copy relocations.
.Pp
.It Cm nodefaultlib
Set the
.Dv DF_1_NODEFLIB
flag to indicate that default library search paths should be ignored.
.Pp
.It Cm nodelete
Set the
.Dv DF_1_NODELETE
flag to indicate that the object cannot be unloaded from a process.
.Pp
.It Cm nodlopen
Set the
.Dv DF_1_NOOPEN
flag to indicate that the object may not be opened by
.Xr dlopen 3 .
.Pp
.It Cm norelro
Do not indicate that portions of the object shold be mapped read-only
after initial relocation processing.
The object will omit the
.Dv PT_GNU_RELRO
segment.
.Pp
.It Cm notext
Allow relocations against read-only segments.
Sets the
.Dv DT_TEXTREL flag in the
.Dv DYNAMIC
section.
.Pp
.It Cm now
Set the
.Dv DF_BIND_NOW
flag to indicate that the run-time loader should perform all relocation
processing as part of object initialization.
By default relocations may be performed on demand.
.Pp
.It Cm origin
Set the
.Dv DF_ORIGIN
flag to indicate that the object requires
$ORIGIN
processing.
.Pp
.It Cm retpolineplt
Emit retpoline format PLT entries as a mitigation for CVE-2017-5715.
.Pp
.It Cm rodynamic
Make the
.Li .dynamic
@ -676,6 +693,18 @@ section read-only.
The
.Dv DT_DEBUG
tag will not be emitted.
.Pp
.It Cm separate-loadable-segments
.It Cm separate-code
.It Cm noseparate-code
Specify whether two adjacent PT_LOAD segments are allowed to overlap in pages.
.Cm noseparate-code
(default) allows overlap.
.Cm separate-code
allows overlap between two executable segments, or two non-executable segments.
.Cm separate-loadable-segments
disallows overlap.
.Pp
.It Cm stack-size Ns = Ns Ar size
Set the main thread's stack size to
.Ar size .
@ -683,9 +712,11 @@ The stack size is recorded as the size of the
.Ar size .
.Dv PT_GNU_STACK
program segment.
.Pp
.It Cm text
Do not allow relocations against read-only segments.
This is the default.
.Pp
.It Cm wxneeded
Create a
.Dv PT_OPENBSD_WXNEEDED

5
lld/test/ELF/fill-trap.s
View File

@ -12,6 +12,11 @@
# RUN: llvm-readobj -l %t | FileCheck %s --check-prefixes=CHECK,PAD
# RUN: od -Ax -x -N16 -j0x1ff0 %t | FileCheck %s --check-prefix=FILL
## -z separate-loadable-segments pads all segments, including the text segment.
# RUN: ld.lld %t.o -z separate-loadable-segments -o %t
# RUN: llvm-readobj -l %t | FileCheck %s --check-prefixes=CHECK,PAD
# RUN: od -Ax -x -N16 -j0x1ff0 %t | FileCheck %s --check-prefix=FILL
# RUN: ld.lld %t.o -z separate-code -z noseparate-code -o %t
# RUN: llvm-readobj -l %t | FileCheck %s --check-prefixes=CHECK,NOPAD

33
lld/test/ELF/separate-segments.s Normal file
View File

@ -0,0 +1,33 @@
# REQUIRES: x86
# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
## -z noseparate-code is the default. All PT_LOAD can have overlapping p_offset
## ranges at runtime.
# RUN: ld.lld -pie %t.o -o %t
# RUN: llvm-readelf -l %t | FileCheck --check-prefix=NONE %s
# NONE: LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x000245 0x000245 R 0x1000
# NONE-NEXT: LOAD 0x000248 0x0000000000001248 0x0000000000001248 0x000001 0x000001 R E 0x1000
# NONE-NEXT: LOAD 0x000250 0x0000000000002250 0x0000000000002250 0x000080 0x000080 RW 0x1000
# NONE-NEXT: LOAD 0x0002d0 0x00000000000032d0 0x00000000000032d0 0x000001 0x000001 RW 0x1000
## -z separate-code makes text segment (RX) separate.
## The two RW can have overlapping p_offset ranges at runtime.
# RUN: ld.lld -pie %t.o -z separate-code -o %t
# RUN: llvm-readelf -l %t | FileCheck --check-prefix=CODE %s
# CODE: LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x000245 0x000245 R 0x1000
# CODE-NEXT: LOAD 0x001000 0x0000000000001000 0x0000000000001000 0x000001 0x000001 R E 0x1000
# CODE-NEXT: LOAD 0x002000 0x0000000000002000 0x0000000000002000 0x000080 0x000080 RW 0x1000
# CODE-NEXT: LOAD 0x002080 0x0000000000003080 0x0000000000003080 0x000001 0x000001 RW 0x1000
## -z separate-loadable-segments makes all segments separate.
# RUN: ld.lld -pie %t.o -z separate-loadable-segments -o %t
# RUN: llvm-readelf -l %t | FileCheck --check-prefix=ALL %s
# ALL: LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x000245 0x000245 R 0x1000
# ALL-NEXT: LOAD 0x001000 0x0000000000001000 0x0000000000001000 0x000001 0x000001 R E 0x1000
# ALL-NEXT: LOAD 0x002000 0x0000000000002000 0x0000000000002000 0x000080 0x000080 RW 0x1000
# ALL-NEXT: LOAD 0x003000 0x0000000000003000 0x0000000000003000 0x000001 0x000001 RW 0x1000
nop
.data
.byte 0