From b8edcdb10f3e4708ee09d7537ffc0ec2fb2b3951 Mon Sep 17 00:00:00 2001 From: "Gustavo A. R. Silva" Date: Thu, 7 May 2020 13:50:57 -0500 Subject: [PATCH] BACKPORT: bpf, libbpf: 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 76497732932f ("cxgb3/l2t: Fix undefined behaviour") Change-Id: I612683a3eca9c84d9c48747ec3015480f64a9fb2 Signed-off-by: Gustavo A. R. Silva Signed-off-by: Daniel Borkmann Acked-by: Yonghong Song Link: https://lore.kernel.org/bpf/20200507185057.GA13981@embeddedor --- kernel/bpf/queue_stack_maps.c | 2 +- tools/lib/bpf/libbpf.c | 2 +- tools/lib/bpf/libbpf_internal.h | 2 +- 3 files changed, 3 insertions(+), 3 deletions(-) diff --git a/kernel/bpf/queue_stack_maps.c b/kernel/bpf/queue_stack_maps.c index 26ba7cb01136..b736078f6a9b 100644 --- a/kernel/bpf/queue_stack_maps.c +++ b/kernel/bpf/queue_stack_maps.c @@ -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) diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c index 98e34c517267..6c678cd04a33 100644 --- a/tools/lib/bpf/libbpf.c +++ b/tools/lib/bpf/libbpf.c @@ -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 { diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h index 92940ae26ada..0dd705a1cb12 100644 --- a/tools/lib/bpf/libbpf_internal.h +++ b/tools/lib/bpf/libbpf_internal.h @@ -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 */