This is a note to let you know that I've just added the patch titled
powerpc/ftrace: Create a dummy stackframe to fix stack unwind
to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
powerpc-ftrace-create-a-dummy-stackframe-to-fix-stack-unwind.patch
and it can be found in the queue-4.14 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From stable+bounces-6799-greg=kroah.com@xxxxxxxxxxxxxxx Fri Dec 15 12:15:27 2023
From: Naveen N Rao <naveen@xxxxxxxxxx>
Date: Fri, 15 Dec 2023 16:41:21 +0530
Subject: powerpc/ftrace: Create a dummy stackframe to fix stack unwind
To: <stable@xxxxxxxxxxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>, Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Message-ID: <20231215111122.2361478-1-naveen@xxxxxxxxxx>
From: Naveen N Rao <naveen@xxxxxxxxxx>
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
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/powerpc/kernel/trace/ftrace_64_mprofile.S | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
--- a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
+++ b/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
@@ -41,6 +41,9 @@ _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)
@@ -64,6 +67,8 @@ _GLOBAL(ftrace_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)
@@ -146,7 +151,7 @@ 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
#ifdef CONFIG_LIVEPATCH
/*
Patches currently in stable-queue which might be from kroah.com@xxxxxxxxxxxxxxx are
queue-4.14/powerpc-ftrace-create-a-dummy-stackframe-to-fix-stack-unwind.patch
queue-4.14/powerpc-ftrace-fix-stack-teardown-in-ftrace_no_trace.patch
