[PATCH 17/17] powerpc/ftrace: Create a dummy stackframe to fix stack unwind
Christophe Leroy
christophe.leroy at csgroup.eu
Fri Jun 23 15:40:34 AEST 2023
Le 19/06/2023 à 11:47, Naveen N Rao a écrit :
> 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. Since this is only relevant when ftrace is active, we nop
> out the instruction to store LR in the LR save area in the profiling
> instruction sequence on ppc32 (and in ppc64 with older gcc versions).
> Instead, in ftrace, we store LR in the LR save area of the previous
> stackframe, and create a minimal stackframe to represent 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. Note that this change now aligns with
> other architectures (arm64, s390, x86).
>
> Signed-off-by: Naveen N Rao <naveen at kernel.org>
Reviewed-by: Christophe Leroy <christophe.leroy at csgroup.eu>
> ---
> arch/powerpc/kernel/trace/ftrace.c | 6 ++++--
> arch/powerpc/kernel/trace/ftrace_entry.S | 11 ++++++++---
> 2 files changed, 12 insertions(+), 5 deletions(-)
>
> diff --git a/arch/powerpc/kernel/trace/ftrace.c b/arch/powerpc/kernel/trace/ftrace.c
> index 82010629cf887c..2956196c98ffdc 100644
> --- a/arch/powerpc/kernel/trace/ftrace.c
> +++ b/arch/powerpc/kernel/trace/ftrace.c
> @@ -229,13 +229,15 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
> /* Expected sequence: 'mflr r0', 'stw r0,4(r1)', 'bl _mcount' */
> ret = ftrace_validate_inst(ip - 8, ppc_inst(PPC_RAW_MFLR(_R0)));
> if (!ret)
> - ret = ftrace_validate_inst(ip - 4, ppc_inst(PPC_RAW_STW(_R0, _R1, 4)));
> + ret = ftrace_modify_code(ip - 4, ppc_inst(PPC_RAW_STW(_R0, _R1, 4)),
> + ppc_inst(PPC_RAW_NOP()));
> } else if (IS_ENABLED(CONFIG_MPROFILE_KERNEL)) {
> /* Expected sequence: 'mflr r0', ['std r0,16(r1)'], 'bl _mcount' */
> ret = ftrace_read_inst(ip - 4, &old);
> if (!ret && !ppc_inst_equal(old, ppc_inst(PPC_RAW_MFLR(_R0)))) {
> ret = ftrace_validate_inst(ip - 8, ppc_inst(PPC_RAW_MFLR(_R0)));
> - ret |= ftrace_validate_inst(ip - 4, ppc_inst(PPC_RAW_STD(_R0, _R1, 16)));
> + ret |= ftrace_modify_code(ip - 4, ppc_inst(PPC_RAW_STD(_R0, _R1, 16)),
> + ppc_inst(PPC_RAW_NOP()));
> }
> } else {
> return -EINVAL;
> diff --git a/arch/powerpc/kernel/trace/ftrace_entry.S b/arch/powerpc/kernel/trace/ftrace_entry.S
> index bab3ab1368a33f..05e981fb526c2e 100644
> --- a/arch/powerpc/kernel/trace/ftrace_entry.S
> +++ b/arch/powerpc/kernel/trace/ftrace_entry.S
> @@ -33,6 +33,11 @@
> * 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)
> + /* Save the original return address in A's stack frame */
> + PPC_STL r0, LRSAVE+STACK_FRAME_MIN_SIZE(r1)
> +
> /* Create our stack frame + pt_regs */
> PPC_STLU r1,-SWITCH_FRAME_SIZE(r1)
>
> @@ -41,8 +46,6 @@
> SAVE_GPRS(3, 10, r1)
>
> #ifdef CONFIG_PPC64
> - /* Save the original return address in A's stack frame */
> - std r0, LRSAVE+SWITCH_FRAME_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 */
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