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Message-ID: <YCgBR6c7UdmhNgwr@krava>
Date: Sat, 13 Feb 2021 17:41:43 +0100
From: Jiri Olsa <jolsa@...hat.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Jiri Olsa <jolsa@...nel.org>,
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 02:21:04PM -0800, Andrii Nakryiko wrote:
> 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?
yes, this is the only place for now, I'll make the change
thanks,
jirka
>
> >
> > /* 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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