Re: [PATCH 5.15.y 1/2] powerpc/ftrace: Create a dummy stackframe to fix stack unwind

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@vger.kernel.org Signed-off-by: Naveen N Rao naveen@kernel.org Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://msgid.link/20230621051349.759567-1-naveen@kernel.org

---

arch/powerpc/kernel/trace/ftrace_64_mprofile.S | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-)

All now queued up, thanks.

greg k-h