Stack trace of csum_partial_copy_generic

Hello Josh,

I'd like to ask you whether objtool is supposed to produce a warning when arch/x86/lib/csum-copy_64.o (produced from arch/x86/lib/csum-copy_64.S). Since I cannot see any specific usage of rbp for defining a stackframe. I'm chasing against poor performance of a network benchmark and this is what perf produces:

# Overhead Command Shared Object Symbol

# ........  ...............  .....................  .............................................
#
    37.30%            iperf  [kernel.kallsyms]      [k] csum_partial_copy_generic                
                      |
                      --- csum_partial_copy_generic
                         |          
                         |--99.98%-- 0x7f809108b7cd
                         |          |          
                         |          |--69.72%-- 0x20000
                         |          |          
                         |           --30.28%-- 0x7f809108b7c2
                         |                     0x20000
                          --0.02%-- [...]

So this is not very helpful in tracing where this is being called from. Presumably somewhere from the networking layer. So should objtool catch this or since csum_partial_copy_generic is a leaf function reliable stack trace isn't needed? Furthermore this function is called from C wrapper in csum-wrappers_64.c - shouldn't at least they be present in the callstack?

This is on 4.6 master from linus and CONFIG_FRAME_POINTER being enabled.

Regards,
Nikolay

Hi Nikolay,

Right, since it's a leaf function, objtool ignores it and lets it do whatever it wants with the frame pointer.

I suspect the problem is that it can't walk the stack because the function overwrites the rbp register. Try replacing all uses of rbp in that function with another register. r15?

(Another solution would be to tell perf to use DWARF unwinding instead of frame pointers, but currently, kernel asm code doesn't have any DWARF annotations. I'm planning on adding support for that soon in the 4.8 timeframe by generating DWARF metadata using objtool.)

Josh