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perf/core: Replace zero-length array with flexible-array 

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 7649773293 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200511201227.GA14041@embeddedor
This commit is contained in:
Gustavo A. R. Silva 2020-05-11 15:12:27 -05:00 committed by Peter Zijlstra
parent 8ac7571a8c
commit c50c75e9b8
3 changed files with 4 additions and 4 deletions
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4
include/linux/perf_event.h
View File

@ -61,7 +61,7 @@ struct perf_guest_info_callbacks {
struct perf_callchain_entry {
__u64 nr;
__u64 ip[0]; /* /proc/sys/kernel/perf_event_max_stack */
__u64 ip[]; /* /proc/sys/kernel/perf_event_max_stack */
};
struct perf_callchain_entry_ctx {
@ -113,7 +113,7 @@ struct perf_raw_record {
struct perf_branch_stack {
__u64 nr;
__u64 hw_idx;
struct perf_branch_entry entries[0];
struct perf_branch_entry entries[];
};
struct task_struct;

2
kernel/events/callchain.c
View File

@ -16,7 +16,7 @@
struct callchain_cpus_entries {
struct rcu_head rcu_head;
struct perf_callchain_entry *cpu_entries[0];
struct perf_callchain_entry *cpu_entries[];
};
int sysctl_perf_event_max_stack __read_mostly = PERF_MAX_STACK_DEPTH;

2
kernel/events/internal.h
View File

@ -55,7 +55,7 @@ struct perf_buffer {
void *aux_priv;
struct perf_event_mmap_page *user_page;
void *data_pages[0];
void *data_pages[];
};
extern void rb_free(struct perf_buffer *rb);