linux-kernel - Re: [PATCH] scripts/faddr2line: Only use text symbols to calculate function size

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Date:   Fri, 6 May 2022 11:34:00 +0530
From:   Kaiwan N Billimoria <kaiwan.billimoria@...il.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] scripts/faddr2line: Only use text symbols to calculate
 function size

Hi Josh,

Unfortunately, it didn't work with my test case (as before):
$ <...>/linux-5.10.60/scripts/faddr2line ./oops_tryv2.ko do_the_work+0x124
bad symbol size: base: 0x0000000000000000 end: 0x0000000000000000
$

What _did_ work was the earlier (much longer) patch you'd sent (your
email dt 20 Jan 2022),
Could you pl recheck...
(As before, i have the test case module here:
https://github.com/PacktPublishing/Linux-Kernel-Debugging/tree/main/ch7/oops_tryv2
)

Regards,
Kaiwan.


On Tue, May 3, 2022 at 12:14 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> With the following commit
>
>   efdb4167e676 ("scripts/faddr2line: Fix "size mismatch" error")
>
> it was discovered that faddr2line can't just read a function's ELF
> size, because that wouldn't match the kallsyms function size which is
> printed in the stack trace.  The kallsyms size includes any padding
> after the function, whereas the ELF size does not.
>
> So faddr2line has to manually calculate the size of a function similar
> to how kallsyms does.  It does so by starting with a sorted list of
> symbols and subtracting the function address from the subsequent
> symbol's address.
>
> That calculation is broken in the case where the function is the last
> (or only) symbol in the .text section, which can occur quite commonly in
> a kernel module or a .o file.  In that case, the next symbol in the
> sorted list might actually be a data symbol, which breaks the function
> size detection:
>
>   $ scripts/faddr2line sound/soundcore.ko sound_devnode+0x5/0x35
>   bad symbol size: base: 0x0000000000000000 end: 0x0000000000000000
>
> Fix it by only including text symbols in the symbol list.
>
> Fixes: efdb4167e676 ("scripts/faddr2line: Fix "size mismatch" error")
> Reported-by: Kaiwan N Billimoria <kaiwan.billimoria@...il.com>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> ---
>  scripts/faddr2line | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/scripts/faddr2line b/scripts/faddr2line
> index 6c6439f69a72..2a130134f1e6 100755
> --- a/scripts/faddr2line
> +++ b/scripts/faddr2line
> @@ -189,7 +189,7 @@ __faddr2line() {
>
>                 DONE=1
>
> -       done < <(${NM} -n $objfile | awk -v fn=$func -v end=$file_end '$3 == fn { found=1; line=$0; start=$1; next } found == 1 { found=0; print line, "0x"$1 } END {if (found == 1) print line, end; }')
> +       done < <(${NM} -n $objfile | awk -v fn=$func -v end=$file_end '$2 !~ /[Tt]/ {next} $3 == fn { found=1; line=$0; start=$1; next } found == 1 { found=0; print line, "0x"$1 } END {if (found == 1) print line, end; }')
>  }
>
>  [[ $# -lt 2 ]] && usage
> --
> 2.34.1
>