On Mon, Nov 6, 2023 at 10:29 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
>
> Martin reported that there is a libbpf complaining of non-zero-value tail
> padding with LIBBPF_OPTS_RESET macro if struct bpf_netkit_opts is modified
> to have a 4-byte tail padding. This only happens to clang compiler.
> The commend line is: ./test_progs -t tc_netkit_multi_links
> Martin and I did some investigation and found this indeed the case and
> the following are the investigation details.
>
> Clang 18:
> clang version 18.0.0
> <I tried clang15/16/17 and they all have similar results>
>
> tools/lib/bpf/libbpf_common.h:
> #define LIBBPF_OPTS_RESET(NAME, ...) \
> do { \
> memset(&NAME, 0, sizeof(NAME)); \
> NAME = (typeof(NAME)) { \
> .sz = sizeof(NAME), \
> __VA_ARGS__ \
> }; \
> } while (0)
>
> #endif
>
> tools/lib/bpf/libbpf.h:
> struct bpf_netkit_opts {
> /* size of this struct, for forward/backward compatibility */
> size_t sz;
> __u32 flags;
> __u32 relative_fd;
> __u32 relative_id;
> __u64 expected_revision;
> size_t :0;
> };
> #define bpf_netkit_opts__last_field expected_revision
> In the above struct bpf_netkit_opts, there is no tail padding.
>
> prog_tests/tc_netkit.c:
> static void serial_test_tc_netkit_multi_links_target(int mode, int target)
> {
> ...
> LIBBPF_OPTS(bpf_netkit_opts, optl);
> ...
> LIBBPF_OPTS_RESET(optl,
> .flags = BPF_F_BEFORE,
> .relative_fd = bpf_program__fd(skel->progs.tc1),
> );
> ...
> }
>
> Let us make the following source change, note that we have a 4-byte
> tailing padding now.
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 6cd9c501624f..0dd83910ae9a 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -803,13 +803,13 @@ bpf_program__attach_tcx(const struct bpf_program *prog, int ifindex,
> struct bpf_netkit_opts {
> /* size of this struct, for forward/backward compatibility */
> size_t sz;
> - __u32 flags;
> __u32 relative_fd;
> __u32 relative_id;
> __u64 expected_revision;
> + __u32 flags;
> size_t :0;
> };
> -#define bpf_netkit_opts__last_field expected_revision
> +#define bpf_netkit_opts__last_field flags
>
> The clang 18 generated asm code looks like below:
> ; LIBBPF_OPTS_RESET(optl,
> 55e3: 48 8d 7d 98 leaq -0x68(%rbp), %rdi
> 55e7: 31 f6 xorl %esi, %esi
> 55e9: ba 20 00 00 00 movl $0x20, %edx
> 55ee: e8 00 00 00 00 callq 0x55f3 <serial_test_tc_netkit_multi_links_target+0x18d3>
> 55f3: 48 c7 85 10 fd ff ff 20 00 00 00 movq $0x20, -0x2f0(%rbp)
> 55fe: 48 8b 85 68 ff ff ff movq -0x98(%rbp), %rax
> 5605: 48 8b 78 18 movq 0x18(%rax), %rdi
> 5609: e8 00 00 00 00 callq 0x560e <serial_test_tc_netkit_multi_links_target+0x18ee>
> 560e: 89 85 18 fd ff ff movl %eax, -0x2e8(%rbp)
> 5614: c7 85 1c fd ff ff 00 00 00 00 movl $0x0, -0x2e4(%rbp)
> 561e: 48 c7 85 20 fd ff ff 00 00 00 00 movq $0x0, -0x2e0(%rbp)
> 5629: c7 85 28 fd ff ff 08 00 00 00 movl $0x8, -0x2d8(%rbp)
> 5633: 48 8b 85 10 fd ff ff movq -0x2f0(%rbp), %rax
> 563a: 48 89 45 98 movq %rax, -0x68(%rbp)
> 563e: 48 8b 85 18 fd ff ff movq -0x2e8(%rbp), %rax
> 5645: 48 89 45 a0 movq %rax, -0x60(%rbp)
> 5649: 48 8b 85 20 fd ff ff movq -0x2e0(%rbp), %rax
> 5650: 48 89 45 a8 movq %rax, -0x58(%rbp)
> 5654: 48 8b 85 28 fd ff ff movq -0x2d8(%rbp), %rax
> 565b: 48 89 45 b0 movq %rax, -0x50(%rbp)
> ; link = bpf_program__attach_netkit(skel->progs.tc2, ifindex, &optl);
>
> At -O0 level, the clang compiler creates an intermediate copy.
> We have below to store 'flags' with 4-byte store and leave another 4 byte
> in the same 8-byte-aligned storage undefined,
> 5629: c7 85 28 fd ff ff 08 00 00 00 movl $0x8, -0x2d8(%rbp)
> and later we store 8-byte to the original zero'ed buffer
> 5654: 48 8b 85 28 fd ff ff movq -0x2d8(%rbp), %rax
> 565b: 48 89 45 b0 movq %rax, -0x50(%rbp)
>
> This caused a problem as the 4-byte value at [%rbp-0x2dc, %rbp-0x2e0)
> may be garbage.
>
> gcc (gcc 11.4) does not have this issue as it does zeroing struct first before
> doing assignments:
> ; LIBBPF_OPTS_RESET(optl,
> 50fd: 48 8d 85 40 fc ff ff leaq -0x3c0(%rbp), %rax
> 5104: ba 20 00 00 00 movl $0x20, %edx
> 5109: be 00 00 00 00 movl $0x0, %esi
> 510e: 48 89 c7 movq %rax, %rdi
> 5111: e8 00 00 00 00 callq 0x5116 <serial_test_tc_netkit_multi_links_target+0x1522>
> 5116: 48 8b 45 f0 movq -0x10(%rbp), %rax
> 511a: 48 8b 40 18 movq 0x18(%rax), %rax
> 511e: 48 89 c7 movq %rax, %rdi
> 5121: e8 00 00 00 00 callq 0x5126 <serial_test_tc_netkit_multi_links_target+0x1532>
> 5126: 48 c7 85 40 fc ff ff 00 00 00 00 movq $0x0, -0x3c0(%rbp)
> 5131: 48 c7 85 48 fc ff ff 00 00 00 00 movq $0x0, -0x3b8(%rbp)
> 513c: 48 c7 85 50 fc ff ff 00 00 00 00 movq $0x0, -0x3b0(%rbp)
> 5147: 48 c7 85 58 fc ff ff 00 00 00 00 movq $0x0, -0x3a8(%rbp)
> 5152: 48 c7 85 40 fc ff ff 20 00 00 00 movq $0x20, -0x3c0(%rbp)
> 515d: 89 85 48 fc ff ff movl %eax, -0x3b8(%rbp)
> 5163: c7 85 58 fc ff ff 08 00 00 00 movl $0x8, -0x3a8(%rbp)
> ; link = bpf_program__attach_netkit(skel->progs.tc2, ifindex, &optl);
>
> It is not clear how to resolve the compiler code generation as the compiler
> generates correct code w.r.t. how to handle unnamed padding in C standard.
> So this patch changed LIBBPF_OPTS_RESET macro to avoid uninitialized tail
> padding. We already knows LIBBPF_OPTS macro works on both gcc and clang,
> even with tail padding. So LIBBPF_OPTS_RESET is changed to be a
> LIBBPF_OPTS followed by a memcpy(), thus avoiding uninitialized tail padding.
>
> The below is asm code generated with this patch and with clang compiler:
> ; LIBBPF_OPTS_RESET(optl,
> 55e3: 48 8d bd 10 fd ff ff leaq -0x2f0(%rbp), %rdi
> 55ea: 31 f6 xorl %esi, %esi
> 55ec: ba 20 00 00 00 movl $0x20, %edx
> 55f1: e8 00 00 00 00 callq 0x55f6 <serial_test_tc_netkit_multi_links_target+0x18d6>
> 55f6: 48 c7 85 10 fd ff ff 20 00 00 00 movq $0x20, -0x2f0(%rbp)
> 5601: 48 8b 85 68 ff ff ff movq -0x98(%rbp), %rax
> 5608: 48 8b 78 18 movq 0x18(%rax), %rdi
> 560c: e8 00 00 00 00 callq 0x5611 <serial_test_tc_netkit_multi_links_target+0x18f1>
> 5611: 89 85 18 fd ff ff movl %eax, -0x2e8(%rbp)
> 5617: c7 85 1c fd ff ff 00 00 00 00 movl $0x0, -0x2e4(%rbp)
> 5621: 48 c7 85 20 fd ff ff 00 00 00 00 movq $0x0, -0x2e0(%rbp)
> 562c: c7 85 28 fd ff ff 08 00 00 00 movl $0x8, -0x2d8(%rbp)
> 5636: 48 8b 85 10 fd ff ff movq -0x2f0(%rbp), %rax
> 563d: 48 89 45 98 movq %rax, -0x68(%rbp)
> 5641: 48 8b 85 18 fd ff ff movq -0x2e8(%rbp), %rax
> 5648: 48 89 45 a0 movq %rax, -0x60(%rbp)
> 564c: 48 8b 85 20 fd ff ff movq -0x2e0(%rbp), %rax
> 5653: 48 89 45 a8 movq %rax, -0x58(%rbp)
> 5657: 48 8b 85 28 fd ff ff movq -0x2d8(%rbp), %rax
> 565e: 48 89 45 b0 movq %rax, -0x50(%rbp)
> ; link = bpf_program__attach_netkit(skel->progs.tc2, ifindex, &optl);
>
> In the above code, a temporary buffer is zeroed and then has proper value assigned.
> Finally, values in temporary buffer are copied to the original variable buffer,
> hence tail padding is guaranteed to be 0.
>
> Cc: Martin KaFai Lau <martin.lau@xxxxxxxxxx>
> Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
> ---
> tools/lib/bpf/libbpf_common.h | 13 ++++++++-----
> 1 file changed, 8 insertions(+), 5 deletions(-)
>
> Changelog:
> v1 -> v2:
> - Do not change the LIBBPF_OPTS_RESET macro definition, rather
> re-implement to avoid potential uninitialized tail padding.
>
> v1 link: https://lore.kernel.org/bpf/20231105185358.1036619-1-yonghong.song@xxxxxxxxx/
>
> diff --git a/tools/lib/bpf/libbpf_common.h b/tools/lib/bpf/libbpf_common.h
> index b7060f254486..ef14e99bc952 100644
> --- a/tools/lib/bpf/libbpf_common.h
> +++ b/tools/lib/bpf/libbpf_common.h
> @@ -79,11 +79,14 @@
> */
> #define LIBBPF_OPTS_RESET(NAME, ...) \
> do { \
> - memset(&NAME, 0, sizeof(NAME)); \
> - NAME = (typeof(NAME)) { \
> - .sz = sizeof(NAME), \
> - __VA_ARGS__ \
> - }; \
> + typeof(NAME) ___##NAME = ({ \
> + memset(&___##NAME, 0, sizeof(typeof(NAME))); \
> + (typeof(NAME)) { \
> + .sz = sizeof(typeof(NAME)), \
> + __VA_ARGS__ \
> + }; \
> + }); \
> + memcpy(&NAME, &___##NAME, sizeof(typeof(NAME))); \
ok, I think this will work in all the situations that LIBBPF_OPTS()
itself works, so looks good.
Just one minor nit: can you please simplify sizeof(typeof(NAME)) ->
sizeof(NAME), it should work without typeof, right?
> } while (0)
>
> #endif /* __LIBBPF_LIBBPF_COMMON_H */
> --
> 2.34.1
>
