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(-)
diff --git a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S b/arch/powerpc/kernel/trace/ftrace_64_mprofile.S index f9fd5f743eba..da2ca0c6c2c4 100644 --- a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S +++ b/arch/powerpc/kernel/trace/ftrace_64_mprofile.S @@ -36,6 +36,9 @@ _GLOBAL(ftrace_regs_caller) /* Save the original return address in A's stack frame */ std r0,LRSAVE(r1)
+ /* Create a minimal stack frame for representing B */ + stdu r1, -STACK_FRAME_MIN_SIZE(r1) + /* Create our stack frame + pt_regs */ stdu r1,-SWITCH_FRAME_SIZE(r1)
@@ -65,6 +68,8 @@ _GLOBAL(ftrace_regs_caller) mflr r7 /* Save it as pt_regs->nip */ std r7, _NIP(r1) + /* Also save it in B's stackframe header for proper unwind */ + std r7, LRSAVE+SWITCH_FRAME_SIZE(r1) /* Save the read LR in pt_regs->link */ std r0, _LINK(r1)
@@ -121,7 +126,7 @@ ftrace_regs_call: ld r2, 24(r1)
/* Pop our stack frame */ - addi r1, r1, SWITCH_FRAME_SIZE + addi r1, r1, SWITCH_FRAME_SIZE+STACK_FRAME_MIN_SIZE
#ifdef CONFIG_LIVEPATCH /* Based on the cmpd above, if the NIP was altered handle livepatch */ @@ -153,6 +158,9 @@ _GLOBAL(ftrace_caller) /* Save the original return address in A's stack frame */ std r0, LRSAVE(r1)
+ /* Create a minimal stack frame for representing B */ + stdu r1, -STACK_FRAME_MIN_SIZE(r1) + /* Create our stack frame + pt_regs */ stdu r1, -SWITCH_FRAME_SIZE(r1)
@@ -166,6 +174,7 @@ _GLOBAL(ftrace_caller) /* Get the _mcount() call site out of LR */ mflr r7 std r7, _NIP(r1) + std r7, LRSAVE+SWITCH_FRAME_SIZE(r1)
/* Save callee's TOC in the ABI compliant location */ std r2, 24(r1) @@ -200,7 +209,7 @@ ftrace_call: ld r2, 24(r1)
/* Pop our stack frame */ - addi r1, r1, SWITCH_FRAME_SIZE + addi r1, r1, SWITCH_FRAME_SIZE+STACK_FRAME_MIN_SIZE
/* Reload original LR */ ld r0, LRSAVE(r1)
base-commit: b50306f77190155d2c14a72be5d2e02254d17dbd