b684c09f09
ppc_save_regs() skips one stack frame while saving the CPU register states.
Instead of saving current R1, it pulls the previous stack frame pointer.
When vmcores caused by direct panic call (such as `echo c >
/proc/sysrq-trigger`), are debugged with gdb, gdb fails to show the
backtrace correctly. On further analysis, it was found that it was because
of mismatch between r1 and NIP.
GDB uses NIP to get current function symbol and uses corresponding debug
info of that function to unwind previous frames, but due to the
mismatching r1 and NIP, the unwinding does not work, and it fails to
unwind to the 2nd frame and hence does not show the backtrace.
GDB backtrace with vmcore of kernel without this patch:
---------
(gdb) bt
#0 0xc0000000002a53e8 in crash_setup_regs (oldregs=<optimized out>,
newregs=0xc000000004f8f8d8) at ./arch/powerpc/include/asm/kexec.h:69
#1 __crash_kexec (regs=<optimized out>) at kernel/kexec_core.c:974
#2 0x0000000000000063 in ?? ()
#3 0xc000000003579320 in ?? ()
---------
Further analysis revealed that the mismatch occurred because
"ppc_save_regs" was saving the previous stack's SP instead of the current
r1. This patch fixes this by storing current r1 in the saved pt_regs.
GDB backtrace with vmcore of patched kernel:
--------
(gdb) bt
#0 0xc0000000002a53e8 in crash_setup_regs (oldregs=0x0, newregs=0xc00000000670b8d8)
at ./arch/powerpc/include/asm/kexec.h:69
#1 __crash_kexec (regs=regs@entry=0x0) at kernel/kexec_core.c:974
#2 0xc000000000168918 in panic (fmt=fmt@entry=0xc000000001654a60 "sysrq triggered crash\n")
at kernel/panic.c:358
#3 0xc000000000b735f8 in sysrq_handle_crash (key=<optimized out>) at drivers/tty/sysrq.c:155
#4 0xc000000000b742cc in __handle_sysrq (key=key@entry=99, check_mask=check_mask@entry=false)
at drivers/tty/sysrq.c:602
#5 0xc000000000b7506c in write_sysrq_trigger (file=<optimized out>, buf=<optimized out>,
count=2, ppos=<optimized out>) at drivers/tty/sysrq.c:1163
#6 0xc00000000069a7bc in pde_write (ppos=<optimized out>, count=<optimized out>,
buf=<optimized out>, file=<optimized out>, pde=0xc00000000362cb40) at fs/proc/inode.c:340
#7 proc_reg_write (file=<optimized out>, buf=<optimized out>, count=<optimized out>,
ppos=<optimized out>) at fs/proc/inode.c:352
#8 0xc0000000005b3bbc in vfs_write (file=file@entry=0xc000000006aa6b00,
buf=buf@entry=0x61f498b4f60 <error: Cannot access memory at address 0x61f498b4f60>,
count=count@entry=2, pos=pos@entry=0xc00000000670bda0) at fs/read_write.c:582
#9 0xc0000000005b4264 in ksys_write (fd=<optimized out>,
buf=0x61f498b4f60 <error: Cannot access memory at address 0x61f498b4f60>, count=2)
at fs/read_write.c:637
#10 0xc00000000002ea2c in system_call_exception (regs=0xc00000000670be80, r0=<optimized out>)
at arch/powerpc/kernel/syscall.c:171
#11 0xc00000000000c270 in system_call_vectored_common ()
at arch/powerpc/kernel/interrupt_64.S:192
--------
Nick adds:
So this now saves regs as though it was an interrupt taken in the
caller, at the instruction after the call to ppc_save_regs, whereas
previously the NIP was there, but R1 came from the caller's caller and
that mismatch is what causes gdb's dwarf unwinder to go haywire.
Signed-off-by: Aditya Gupta <adityag@linux.ibm.com>
Fixes: d16a58f885 ("powerpc: Improve ppc_save_regs()")
Reivewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230615091047.90433-1-adityag@linux.ibm.com
54 lines
1.3 KiB
ArmAsm
54 lines
1.3 KiB
ArmAsm
/* SPDX-License-Identifier: GPL-2.0-or-later */
|
|
/*
|
|
* Copyright (C) 1996 Paul Mackerras.
|
|
*
|
|
* NOTE: assert(sizeof(buf) > 23 * sizeof(long))
|
|
*/
|
|
#include <asm/processor.h>
|
|
#include <asm/ppc_asm.h>
|
|
#include <asm/asm-offsets.h>
|
|
#include <asm/ptrace.h>
|
|
#include <asm/asm-compat.h>
|
|
|
|
/*
|
|
* Grab the register values as they are now.
|
|
* This won't do a particularly good job because we really
|
|
* want our caller's caller's registers, and our caller has
|
|
* already executed its prologue.
|
|
* ToDo: We could reach back into the caller's save area to do
|
|
* a better job of representing the caller's state (note that
|
|
* that will be different for 32-bit and 64-bit, because of the
|
|
* different ABIs, though).
|
|
*/
|
|
_GLOBAL(ppc_save_regs)
|
|
/* This allows stack frame accessor macros and offsets to be used */
|
|
subi r3,r3,STACK_INT_FRAME_REGS
|
|
PPC_STL r0,GPR0(r3)
|
|
#ifdef CONFIG_PPC32
|
|
stmw r2,GPR2(r3)
|
|
#else
|
|
SAVE_GPRS(2, 31, r3)
|
|
lbz r0,PACAIRQSOFTMASK(r13)
|
|
PPC_STL r0,SOFTE(r3)
|
|
#endif
|
|
/* store current SP */
|
|
PPC_STL r1,GPR1(r3)
|
|
/* get caller's LR */
|
|
PPC_LL r4,0(r1)
|
|
PPC_LL r0,LRSAVE(r4)
|
|
PPC_STL r0,_LINK(r3)
|
|
mflr r0
|
|
PPC_STL r0,_NIP(r3)
|
|
mfmsr r0
|
|
PPC_STL r0,_MSR(r3)
|
|
mfctr r0
|
|
PPC_STL r0,_CTR(r3)
|
|
mfxer r0
|
|
PPC_STL r0,_XER(r3)
|
|
mfcr r0
|
|
PPC_STL r0,_CCR(r3)
|
|
li r0,0
|
|
PPC_STL r0,_TRAP(r3)
|
|
PPC_STL r0,ORIG_GPR3(r3)
|
|
blr
|