commit b684c09f09e7a6af3794d4233ef785819e72db79 upstream.

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: d16a58f8854b1 ("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

Cc: stable@vger.kernel.org Signed-off-by: Aditya Gupta adityag@linux.ibm.com ---

This is backport for the upstream commit b684c09f09e7a6af3794d4233ef785819e72db79. This solves a register mismatch issue while saving registers after a kernel crash. With this fixed, gdb can also unwind backtraces correctly for vmcores collected from such kernel crashes.

Please let me know if this is not correct format for a backport patch. Thanks.

--- --- arch/powerpc/kernel/ppc_save_regs.S | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kernel/ppc_save_regs.S b/arch/powerpc/kernel/ppc_save_regs.S index 2d4d21bb46a9..5d284f78b0b4 100644 --- a/arch/powerpc/kernel/ppc_save_regs.S +++ b/arch/powerpc/kernel/ppc_save_regs.S @@ -58,10 +58,10 @@ _GLOBAL(ppc_save_regs) lbz r0,PACAIRQSOFTMASK(r13) PPC_STL r0,SOFTE-STACK_FRAME_OVERHEAD(r3) #endif - /* go up one stack frame for SP */ - PPC_LL r4,0(r1) - PPC_STL r4,1*SZL(r3) + /* store current SP */ + PPC_STL r1,1*SZL(r3) /* get caller's LR */ + PPC_LL r4,0(r1) PPC_LL r0,LRSAVE(r4) PPC_STL r0,_LINK-STACK_FRAME_OVERHEAD(r3) mflr r0