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BACKPORT: bpf, libbpf: Replace zero-length array with flexible-array

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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")

Change-Id: I612683a3eca9c84d9c48747ec3015480f64a9fb2
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200507185057.GA13981@embeddedor
This commit is contained in:
Gustavo A. R. Silva 2020-05-07 13:50:57 -05:00 • committed by basamaryan
commit b8edcdb10f
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3 changed files with 3 additions and 3 deletions
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2
kernel/bpf/queue_stack_maps.c
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@ -19,7 +19,7 @@ struct bpf_queue_stack {
u32 head, tail;
u32 size; /* max_entries + 1 */
char elements[0] __aligned(8);
char elements[] __aligned(8);
};
static struct bpf_queue_stack *bpf_queue_stack(struct bpf_map *map)

2
tools/lib/bpf/libbpf.c
View file

@ -5593,7 +5593,7 @@ error:
struct perf_sample_raw {
struct perf_event_header header;
uint32_t size;
char data[0];
char data[];
};
struct perf_sample_lost {

2
tools/lib/bpf/libbpf_internal.h
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@ -104,7 +104,7 @@ struct btf_ext_info_sec {
__u32 sec_name_off;
__u32 num_info;
/* Followed by num_info * record_size number of bytes */
__u8 data[0];
__u8 data[];
};
/* The minimum bpf_func_info checked by the loader */