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SubjectRe: [PATCH bpf-next 2/3] ftrace: Keep address offset in ftrace_lookup_symbols
On Thu, Jun 02, 2022 at 03:52:03PM -0700, Andrii Nakryiko wrote:
> On Fri, May 27, 2022 at 1:56 PM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > We want to store the resolved address on the same index as
> > the symbol string, because that's the user (bpf kprobe link)
> > code assumption.
> >
> > Also making sure we don't store duplicates that might be
> > present in kallsyms.
> >
> > Fixes: bed0d9a50dac ("ftrace: Add ftrace_lookup_symbols function")
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> > kernel/trace/ftrace.c | 13 +++++++++++--
> > 1 file changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index 674add0aafb3..00d0ba6397ed 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -7984,15 +7984,23 @@ static int kallsyms_callback(void *data, const char *name,
> > struct module *mod, unsigned long addr)
> > {
> > struct kallsyms_data *args = data;
> > + const char **sym;
> > + int idx;
> >
> > - if (!bsearch(&name, args->syms, args->cnt, sizeof(*args->syms), symbols_cmp))
> > + sym = bsearch(&name, args->syms, args->cnt, sizeof(*args->syms), symbols_cmp);
> > + if (!sym)
> > + return 0;
> > +
> > + idx = sym - args->syms;
> > + if (args->addrs[idx])
>
> if we have duplicated symbols we won't increment args->found here,
> right? So we won't stop early. But we also don't want to increment
> args->found here because we use it to check that we don't have
> duplicates (in addition to making sure we resolved all the unique
> symbols), right?
>
> So I wonder if in this situation should we return some error code to
> signify that we encountered symbol duplicate?

hum, this callback is called for each kallsyms symbol and there
are duplicates in /proc/kallsyms.. so even if we have just single
copy of such symbol in args->syms, bsearch will find this single
symbol for all the duplicates in /proc/kallsyms and we will endup
in here.. and it's still fine, we should continue

jirka

>
>
> > return 0;
> >
> > addr = ftrace_location(addr);
> > if (!addr)
> > return 0;
> >
> > - args->addrs[args->found++] = addr;
> > + args->addrs[idx] = addr;
> > + args->found++;
> > return args->found == args->cnt ? 1 : 0;
> > }
> >
> > @@ -8017,6 +8025,7 @@ int ftrace_lookup_symbols(const char **sorted_syms, size_t cnt, unsigned long *a
> > struct kallsyms_data args;
> > int err;
> >
> > + memset(addrs, 0x0, sizeof(*addrs) * cnt);
> > args.addrs = addrs;
> > args.syms = sorted_syms;
> > args.cnt = cnt;
> > --
> > 2.35.3
> >

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