6.6-stable review patch. If anyone has any objections, please let me know.
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From: Yonghong Song yonghong.song@linux.dev
[ Upstream commit 7f7c43693c1b46652cfafb7af67ba31726d6ec4e ]
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: 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.
Signed-off-by: Yonghong Song yonghong.song@linux.dev Signed-off-by: Andrii Nakryiko andrii@kernel.org Tested-by: Martin KaFai Lau martin.lau@kernel.org Link: https://lore.kernel.org/bpf/20231107201511.2548645-1-yonghong.song@linux.dev Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- tools/lib/bpf/libbpf_common.h | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-)
--- 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(NAME)); \ + (typeof(NAME)) { \ + .sz = sizeof(NAME), \ + __VA_ARGS__ \ + }; \ + }); \ + memcpy(&NAME, &___##NAME, sizeof(NAME)); \ } while (0)
#endif /* __LIBBPF_LIBBPF_COMMON_H */