On Fri, Dec 15, 2023 at 04:44:32PM +0530, Naveen N Rao wrote:
> commit 41a506ef71eb38d94fe133f565c87c3e06ccc072 upstream.
>
> With ppc64 -mprofile-kernel and ppc32 -pg, profiling instructions to
> call into ftrace are emitted right at function entry. The instruction
> sequence used is minimal to reduce overhead. Crucially, a stackframe is
> not created for the function being traced. This breaks stack unwinding
> since the function being traced does not have a stackframe for itself.
> As such, it never shows up in the backtrace:
>
> /sys/kernel/debug/tracing # echo 1 > /proc/sys/kernel/stack_tracer_enabled
> /sys/kernel/debug/tracing # cat stack_trace
> Depth Size Location (17 entries)
> ----- ---- --------
> 0) 4144 32 ftrace_call+0x4/0x44
> 1) 4112 432 get_page_from_freelist+0x26c/0x1ad0
> 2) 3680 496 __alloc_pages+0x290/0x1280
> 3) 3184 336 __folio_alloc+0x34/0x90
> 4) 2848 176 vma_alloc_folio+0xd8/0x540
> 5) 2672 272 __handle_mm_fault+0x700/0x1cc0
> 6) 2400 208 handle_mm_fault+0xf0/0x3f0
> 7) 2192 80 ___do_page_fault+0x3e4/0xbe0
> 8) 2112 160 do_page_fault+0x30/0xc0
> 9) 1952 256 data_access_common_virt+0x210/0x220
> 10) 1696 400 0xc00000000f16b100
> 11) 1296 384 load_elf_binary+0x804/0x1b80
> 12) 912 208 bprm_execve+0x2d8/0x7e0
> 13) 704 64 do_execveat_common+0x1d0/0x2f0
> 14) 640 160 sys_execve+0x54/0x70
> 15) 480 64 system_call_exception+0x138/0x350
> 16) 416 416 system_call_common+0x160/0x2c4
>
> Fix this by having ftrace create a dummy stackframe for the function
> being traced. With this, backtraces now capture the function being
> traced:
>
> /sys/kernel/debug/tracing # cat stack_trace
> Depth Size Location (17 entries)
> ----- ---- --------
> 0) 3888 32 _raw_spin_trylock+0x8/0x70
> 1) 3856 576 get_page_from_freelist+0x26c/0x1ad0
> 2) 3280 64 __alloc_pages+0x290/0x1280
> 3) 3216 336 __folio_alloc+0x34/0x90
> 4) 2880 176 vma_alloc_folio+0xd8/0x540
> 5) 2704 416 __handle_mm_fault+0x700/0x1cc0
> 6) 2288 96 handle_mm_fault+0xf0/0x3f0
> 7) 2192 48 ___do_page_fault+0x3e4/0xbe0
> 8) 2144 192 do_page_fault+0x30/0xc0
> 9) 1952 608 data_access_common_virt+0x210/0x220
> 10) 1344 16 0xc0000000334bbb50
> 11) 1328 416 load_elf_binary+0x804/0x1b80
> 12) 912 64 bprm_execve+0x2d8/0x7e0
> 13) 848 176 do_execveat_common+0x1d0/0x2f0
> 14) 672 192 sys_execve+0x54/0x70
> 15) 480 64 system_call_exception+0x138/0x350
> 16) 416 416 system_call_common+0x160/0x2c4
>
> This results in two additional stores in the ftrace entry code, but
> produces reliable backtraces.
>
> Fixes: 153086644fd1 ("powerpc/ftrace: Add support for -mprofile-kernel ftrace ABI")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Naveen N Rao <naveen@xxxxxxxxxx>
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Link: https://msgid.link/20230621051349.759567-1-naveen@xxxxxxxxxx
> ---
> arch/powerpc/kernel/trace/ftrace_64_mprofile.S | 13 +++++++++++--
> 1 file changed, 11 insertions(+), 2 deletions(-)
>
All now queued up, thanks.
greg k-h
