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
This is a common offset that currently uses the overloaded
STACK_FRAME_OVERHEAD constant. It's easier to read and more
flexible to use a specific regs offset for this.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221127124942.1665522-8-npiggin@gmail.com
Based on 1 normalized pattern(s):
this program is free software you can redistribute it and or modify
it under the terms of the gnu general public license as published by
the free software foundation either version 2 of the license or at
your option any later version
extracted by the scancode license scanner the SPDX license identifier
GPL-2.0-or-later
has been chosen to replace the boilerplate/reference in 3029 file(s).
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Allison Randal <allison@lohutok.net>
Cc: linux-spdx@vger.kernel.org
Link: https://lkml.kernel.org/r/20190527070032.746973796@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This patch moves ASM_CONST() and stringify_in_c() into
dedicated asm-const.h, then cleans all related inclusions.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: asm-compat.h should include asm-const.h]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
arch/powerpc/Makefile activates -mmultiple on BE PPC32 configs
in order to use multiple word instructions in functions entry/exit.
The patch does the same for the asm parts, for consistency.
On processors like the 8xx on which insn fetching is pretty slow,
this speeds up registers save/restore.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: PPC32 is BE only, so drop the endian checks]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Since STACK_FRAME_OVERHEAD is defined in asm/ptrace.h and that
is ASSEMBER safe, we can just include that instead of going via
asm-offsets.h.
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Today the arch/powerpc/xmon/setjmp.S file contains only the
xmon_save_regs function. We want to use it for kdump purposes, so
let's move the file into arch/powerpc/kernel/ and give the function a
more generic name (ppc_save_regs).
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Aditya Gupta