Message-ID: <CAEf4BzYN7FnGjEYMDqQFK1LryUi0+cBTqaFXmPU_kBN1jJ+LLg@mail.gmail.com>
Date:   Fri, 12 Feb 2021 14:21:04 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>, dwarves@...r.kernel.org,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Yonghong Song <yhs@...com>, Hao Luo <haoluo@...gle.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Sedat Dilek <sedat.dilek@...il.com>
Subject: Re: [PATCH] btf_encoder: Match ftrace addresses within elf functions

On Fri, Feb 12, 2021 at 2:05 PM Jiri Olsa <jolsa@...nel.org> wrote:
>
> Currently when processing DWARF function, we check its entrypoint
> against ftrace addresses, assuming that the ftrace address matches
> with function's entrypoint.
>
> This is not the case on some architectures as reported by Nathan
> when building kernel on arm [1].
>
> Fixing the check to take into account the whole function not
> just the entrypoint.
>
> Most of the is_ftrace_func code was contributed by Andrii.
>
> [1] https://lore.kernel.org/bpf/20210209034416.GA1669105@ubuntu-m3-large-x86/
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---

LGTM. But see another suggestion below. In either case:

Acked-by: Andrii Nakryiko <andrii@...nel.org>

>  btf_encoder.c | 55 +++++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 45 insertions(+), 10 deletions(-)
>
> diff --git a/btf_encoder.c b/btf_encoder.c
> index b124ec20a689..03242f04c55d 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -36,6 +36,7 @@ struct funcs_layout {
>  struct elf_function {
>         const char      *name;
>         unsigned long    addr;
> +       unsigned long    size;
>         unsigned long    sh_addr;
>         bool             generated;
>  };
> @@ -98,6 +99,7 @@ static int collect_function(struct btf_elf *btfe, GElf_Sym *sym,
>
>         functions[functions_cnt].name = name;
>         functions[functions_cnt].addr = elf_sym__value(sym);
> +       functions[functions_cnt].size = elf_sym__size(sym);
>         functions[functions_cnt].sh_addr = sh.sh_addr;
>         functions[functions_cnt].generated = false;
>         functions_cnt++;
> @@ -236,6 +238,48 @@ get_kmod_addrs(struct btf_elf *btfe, __u64 **paddrs, __u64 *pcount)
>         return 0;
>  }
>
> +static int is_ftrace_func(struct elf_function *func, __u64 *addrs,
> +                         __u64 count, bool kmod)
> +{
> +       /*
> +        * For vmlinux image both addrs[x] and functions[x]::addr
> +        * values are final address and are comparable.
> +        *
> +        * For kernel module addrs[x] is final address, but
> +        * functions[x]::addr is relative address within section
> +        * and needs to be relocated by adding sh_addr.
> +        */
> +       __u64 start = kmod ? func->addr + func->sh_addr : func->addr;
> +       __u64 addr, end = func->addr + func->size;
> +
> +       /*
> +        * The invariant here is addr[r] that is the smallest address
> +        * that is >= than function start addr. Except the corner case
> +        * where there is no such r, but for that we have a final check
> +        * in the return.
> +        */
> +       size_t l = 0, r = count - 1, m;
> +
> +       /* make sure we don't use invalid r */
> +       if (count == 0)
> +               return false;
> +
> +       while (l < r) {
> +               m = l + (r - l) / 2;
> +               addr = addrs[m];
> +
> +               if (addr >= start) {
> +                       /* we satisfy invariant, so tighten r */
> +                       r = m;
> +               } else {
> +                       /* m is not good enough as l, maybe m + 1 will be */
> +                       l = m + 1;
> +               }
> +       }
> +
> +       return start <= addrs[r] && addrs[r] < end;
> +}
> +
>  static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
>  {
>         __u64 *addrs, count, i;
> @@ -275,18 +319,9 @@ static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
>          */
>         for (i = 0; i < functions_cnt; i++) {
>                 struct elf_function *func = &functions[i];
> -               /*
> -                * For vmlinux image both addrs[x] and functions[x]::addr
> -                * values are final address and are comparable.
> -                *
> -                * For kernel module addrs[x] is final address, but
> -                * functions[x]::addr is relative address within section
> -                * and needs to be relocated by adding sh_addr.
> -                */
> -               __u64 addr = kmod ? func->addr + func->sh_addr : func->addr;

if we just...

if (kmod)
    func->addr += func->sh_addr;

... here, that would make is_ftrace_func() free of kmod knowledge. If
there are other places that rely on kmod vs non-kmod address of a
function, that would be simplified as well, right?

>
>                 /* Make sure function is within ftrace addresses. */
> -               if (bsearch(&addr, addrs, count, sizeof(addrs[0]), addrs_cmp)) {
> +               if (is_ftrace_func(func, addrs, count, kmod)) {
>                         /*
>                          * We iterate over sorted array, so we can easily skip
>                          * not valid item and move following valid field into
> --
> 2.29.2
>

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