netdev - Re: [PATCHv2] btf_encoder: Match ftrace addresses within elf functions

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Message-ID: <YC0PwGjiccm5zGrI@kernel.org>
Date:   Wed, 17 Feb 2021 09:44:48 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Sedat Dilek <sedat.dilek@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Andrii Nakryiko <andrii@...nel.org>, dwarves@...r.kernel.org,
        netdev@...r.kernel.org, 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>
Subject: Re: [PATCHv2] btf_encoder: Match ftrace addresses within elf
 functions

Em Sat, Feb 13, 2021 at 08:11:40PM +0100, Sedat Dilek escreveu:
> On Sat, Feb 13, 2021 at 5:46 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/
> > Acked-by: Andrii Nakryiko <andrii@...nel.org>
> > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> 
> Tested this v2 together with "btf_encoder: sanitize non-regular int
> base type" v2 on top of pahole v1.20
> 
> Tested-by: Sedat Dilek <sedat.dilek@...il.com> # Linux v5.11-rc7+ and
> LLVM/Clang v12.0.0-rc1 on x86 (64bit)

Thanks, added to the cset,

- Arnaldo
 
> - Sedat -
> 
> > ---
> > v2 changes:
> >   - update functions addr directly [Andrii]
> >
> >  btf_encoder.c | 40 ++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 38 insertions(+), 2 deletions(-)
> >
> > diff --git a/btf_encoder.c b/btf_encoder.c
> > index b124ec20a689..80e896961d4e 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 +238,39 @@ 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)
> > +{
> > +       __u64 start = 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;
> > @@ -283,10 +318,11 @@ static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
> >                  * 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 (kmod)
> > +                       func->addr += func->sh_addr;
> >
> >                 /* Make sure function is within ftrace addresses. */
> > -               if (bsearch(&addr, addrs, count, sizeof(addrs[0]), addrs_cmp)) {
> > +               if (is_ftrace_func(func, addrs, count)) {
> >                         /*
> >                          * We iterate over sorted array, so we can easily skip
> >                          * not valid item and move following valid field into
> > --
> > 2.29.2
> >

-- 

- Arnaldo