[PATCH 6.1 089/127] powerpc/ftrace: Create a dummy stackframe to fix stack unwind

Greg Kroah-Hartman
2023-08-09T10:55:22Z

From: Naveen N Rao <naveen@kernel.org>

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/powerpc/kernel/trace/ftrace_mprofile.S | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

--- a/arch/powerpc/kernel/trace/ftrace_mprofile.S
+++ b/arch/powerpc/kernel/trace/ftrace_mprofile.S
@@ -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 */