From: Alessandro Carminati <redacted>
Date: Fri, 21 Jul 2023 14:40:54 +0200

Hello,

I apologize for being noisy today.

In an effort to be collaborative, I would like to share my thoughts on why I
see duplicate symbols in fs/binfmt_elf.c.

[...]

~ $ nm -n built-in.a | grep set_brk -B100| egrep "set_brk|\.o:$"
fs/binfmt_elf.o:
00000000000000d4 t set_brk
fs/compat_binfmt_elf.o:
00000000000000d4 t set_brk

These two symbols come from fs/binfmt_elf.o and fs/compat_binfmt_elf.o, and
they are just two symbols that happen to share the same name, as is common
in the kernel.

at the same time, addr2line reports symbols to be generated from the same file.

~ $ llvm-addr2line-14 -fe vmlinux ffff8000082f1d2c ffff8000082f4454
set_brk
/home/alessandro/src/lka64/linux-6.4/fs/binfmt_elf.c:114
set_brk
/home/alessandro/src/lka64/linux-6.4/fs/binfmt_elf.c:114
~ $
~ $ addr2line -fe vmlinux ffff8000082f1d2c ffff8000082f4454
set_brk
/home/alessandro/src/lka64/linux-6.4/fs/binfmt_elf.c:114
set_brk
/home/alessandro/src/lka64/linux-6.4/fs/binfmt_elf.c:114

looking at the source code:
https://elixir.bootlin.com/linux/v6.4/source/fs/compat_binfmt_elf.c#L144
the cause of this behavior, which is unexpected but correct.

Oh man, that's some deep investigation!
Let me go back to my old tree I was sending RFC from and check that
particular case there...

$ grep set_brk test_ksyms
ffffffff814b46c0 t fs/binfmt_elf.c:set_brk
ffffffff814b7040 t fs/compat_binfmt_elf.c:set_brk

Looks a bit better :D

The rationale is that using source file + line number iproduces better kallsyms
table, but it is still do not produce unique names.

BR
Alessandro

quoted

As a final note, please understand that my patch was not intended to undermine
anyone's work. I simply encountered a problem that I wanted to help solve.
Attached to this message is my code, in case anyone wants to replicate it.
I would appreciate being kept in the loop, as I genuinely want to assist in
fixing this problem.

Thanks,
Olek