linux-kernel - Re: [PATCH v1] bpftool: Align output skeleton ELF code

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Message-ID: <0f367aff-b818-d59b-513a-cadd6fa67782@oracle.com>
Date:   Tue, 3 Oct 2023 09:58:09 +0100
From:   Alan Maguire <alan.maguire@...cle.com>
To:     Ian Rogers <irogers@...gle.com>,
        Quentin Monnet <quentin@...valent.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] bpftool: Align output skeleton ELF code

On 03/10/2023 05:29, Ian Rogers wrote:
> On Mon, Oct 2, 2023 at 3:32 PM Ian Rogers <irogers@...gle.com> wrote:
>>
>> libbpf accesses the ELF data requiring at least 8 byte alignment,
>> however, the data is generated into a C string that doesn't guarantee
>> alignment. Fix this by assigning to an aligned char array, use sizeof
>> on the array, less one for the \0 terminator.
>>
>> Signed-off-by: Ian Rogers <irogers@...gle.com>

this looks like a great catch to me!

Reviewed-by: Alan Maguire <alan.maguire@...cle.com>

>> ---
> 
> Perhaps this could have a fixes tag:
> Fixes: d510296d331a ("bpftool: Use syscall/loader program in "prog
> load" and "gen skeleton" command.")
> 

Yep, or perhaps

Fixes: a6cc6b34b93e ("bpftool: Provide a helper method for accessing
skeleton's embedded ELF data")



> The unaligned problem was seen in perf's offcpu code as well as bcc's
> libbpf_tools. I didn't see problems with map data and opts data, but
> inspection of the code shows they likely have the same issue. I was
> testing with -fsanitize=alignment and
> -fsanitize-undefined-trap-on-error.
> 
> Thanks,
> Ian
> 
>>  tools/bpf/bpftool/gen.c | 15 +++++++++------
>>  1 file changed, 9 insertions(+), 6 deletions(-)
>>
>> diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
>> index 2883660d6b67..b8ebcee9bc56 100644
>> --- a/tools/bpf/bpftool/gen.c
>> +++ b/tools/bpf/bpftool/gen.c
>> @@ -1209,7 +1209,7 @@ static int do_skeleton(int argc, char **argv)
>>         codegen("\
>>                 \n\
>>                                                                             \n\
>> -                       s->data = (void *)%2$s__elf_bytes(&s->data_sz);     \n\
>> +                       s->data = (void *)%1$s__elf_bytes(&s->data_sz);     \n\
>>                                                                             \n\
>>                         obj->skeleton = s;                                  \n\
>>                         return 0;                                           \n\
>> @@ -1218,12 +1218,12 @@ static int do_skeleton(int argc, char **argv)
>>                         return err;                                         \n\
>>                 }                                                           \n\
>>                                                                             \n\
>> -               static inline const void *%2$s__elf_bytes(size_t *sz)       \n\
>> +               static inline const void *%1$s__elf_bytes(size_t *sz)       \n\
>>                 {                                                           \n\
>> -                       *sz = %1$d;                                         \n\
>> -                       return (const void *)\"\\                           \n\
>> -               "
>> -               , file_sz, obj_name);
>> +                       static const char data[] __attribute__((__aligned__(8))) = \"\\\n\
>> +               ",
>> +               obj_name
>> +       );
>>
>>         /* embed contents of BPF object file */
>>         print_hex(obj_data, file_sz);
>> @@ -1231,6 +1231,9 @@ static int do_skeleton(int argc, char **argv)
>>         codegen("\
>>                 \n\
>>                 \";                                                         \n\
>> +                                                                           \n\
>> +                       *sz = sizeof(data) - 1;                             \n\
>> +                       return (const void *)data;                          \n\
>>                 }                                                           \n\
>>                                                                             \n\
>>                 #ifdef __cplusplus                                          \n\
>> --
>> 2.42.0.582.g8ccd20d70d-goog
>>
>