[RFC PATCH 1/1] powerpc: update ppc_save_regs to save current r1 in pt_regs

[Aditya Gupta]

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 SP (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 SP 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 at entry=0x0) at kernel/kexec_core.c:974
 #2  0xc000000000168918 in panic (fmt=fmt at 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 at entry=99, check_mask=check_mask at 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 at entry=0xc000000006aa6b00,
    buf=buf at entry=0x61f498b4f60 <error: Cannot access memory at address 0x61f498b4f60>,
    count=count at entry=2, pos=pos at 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
--------

Signed-off-by: Aditya Gupta <adityag at linux.ibm.com>
---

More information:

This problem with gdb backtrace was discovered while working on a crash
tool enhancement to improve crash analysis using gdb passthrough to be
able print function arguments and local variables inside crash tool. gdb
passthrough simply asks gdb to handle the backtrace printing, where it
was noticed that it could not print correct backtrace in some vmcores.

The changes introduced here has an implication on xmon, that it will show
one extra `xmon` frame in backtrace. By looking at older commits it seems
that originally the ppc_save_regs function was introduced as
xmon_save_regs(). But now the same function has been renamed to
ppc_save_regs() and been used in few other places as well.

Is it still needed to skip one stack frame except if the caller is xmon() ?
If I am correct it was skipping one stack frame to avoid displaying xmon()
stack frame in backtrace. Please advice me if my understanding is correct or not.

---
 arch/powerpc/kernel/ppc_save_regs.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/ppc_save_regs.S b/arch/powerpc/kernel/ppc_save_regs.S
index 49813f982468..0caae1c8cf99 100644
--- a/arch/powerpc/kernel/ppc_save_regs.S
+++ b/arch/powerpc/kernel/ppc_save_regs.S
@@ -31,8 +31,8 @@ _GLOBAL(ppc_save_regs)
 	lbz	r0,PACAIRQSOFTMASK(r13)
 	PPC_STL	r0,SOFTE(r3)
 #endif
-	/* go up one stack frame for SP */
-	PPC_LL	r4,0(r1)
+	/* store current SP */
+	mr	r4,r1
 	PPC_STL	r4,GPR1(r3)
 	/* get caller's LR */
 	PPC_LL	r0,LRSAVE(r4)
-- 
2.40.1