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powerpc/ftrace: Create a dummy stackframe to fix stack unwind

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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: 153086644f ("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
This commit is contained in:
Naveen N Rao 2023-06-21 10:43:49 +05:30 committed by Michael Ellerman
parent 6722b25712
commit 41a506ef71
1 changed files with 7 additions and 2 deletions
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9
arch/powerpc/kernel/trace/ftrace_mprofile.S
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@ -33,6 +33,9 @@
* and then arrange for the ftrace function to be called.
*/
.macro ftrace_regs_entry allregs
/* Create a minimal stack frame for representing B */
PPC_STLU r1, -STACK_FRAME_MIN_SIZE(r1)
/* Create our stack frame + pt_regs */
PPC_STLU r1,-SWITCH_FRAME_SIZE(r1)
@ -42,7 +45,7 @@
#ifdef CONFIG_PPC64
/* Save the original return address in A's stack frame */
std r0, LRSAVE+SWITCH_FRAME_SIZE(r1)
std r0, LRSAVE+SWITCH_FRAME_SIZE+STACK_FRAME_MIN_SIZE(r1)
/* Ok to continue? */
lbz r3, PACA_FTRACE_ENABLED(r13)
cmpdi r3, 0
@ -77,6 +80,8 @@
mflr r7
/* Save it as pt_regs->nip */
PPC_STL r7, _NIP(r1)
/* Also save it in B's stackframe header for proper unwind */
PPC_STL r7, LRSAVE+SWITCH_FRAME_SIZE(r1)
/* Save the read LR in pt_regs->link */
PPC_STL r0, _LINK(r1)
@ -142,7 +147,7 @@
#endif
/* Pop our stack frame */
addi r1, r1, SWITCH_FRAME_SIZE
addi r1, r1, SWITCH_FRAME_SIZE+STACK_FRAME_MIN_SIZE
#ifdef CONFIG_LIVEPATCH_64
/* Based on the cmpd above, if the NIP was altered handle livepatch */