powerpc/vdso: Fix incorrect CFI in gettimeofday.S

As reported by Alan, the CFI (Call Frame Information) in the VDSO time
routines is incorrect since commit ce7d8056e38b ("powerpc/vdso: Prepare
for switching VDSO to generic C implementation.").

DWARF has a concept called the CFA (Canonical Frame Address), which on
powerpc is calculated as an offset from the stack pointer (r1). That
means when the stack pointer is changed there must be a corresponding
CFI directive to update the calculation of the CFA.

The current code is missing those directives for the changes to r1,
which prevents gdb from being able to generate a backtrace from inside
VDSO functions, eg:

  Breakpoint 1, 0x00007ffff7f804dc in __kernel_clock_gettime ()
  (gdb) bt
  #0  0x00007ffff7f804dc in __kernel_clock_gettime ()
  #1  0x00007ffff7d8872c in clock_gettime@@GLIBC_2.17 () from /lib64/libc.so.6
  #2  0x00007fffffffd960 in ?? ()
  #3  0x00007ffff7d8872c in clock_gettime@@GLIBC_2.17 () from /lib64/libc.so.6
  Backtrace stopped: frame did not save the PC

Alan helpfully describes some rules for correctly maintaining the CFI information:

  1) Every adjustment to the current frame address reg (ie. r1) must be
     described, and exactly at the instruction where r1 changes. Why?
     Because stack unwinding might want to access previous frames.

  2) If a function changes LR or any non-volatile register, the save
     location for those regs must be given. The CFI can be at any
     instruction after the saves up to the point that the reg is
     changed.
     (Exception: LR save should be described before a bl. not after)

  3) If asychronous unwind info is needed then restores of LR and
     non-volatile regs must also be described. The CFI can be at any
     instruction after the reg is restored up to the point where the
     save location is (potentially) trashed.

Fix the inability to backtrace by adding CFI directives describing the
changes to r1, ie. satisfying rule 1.

Also change the information for LR to point to the copy saved on the
stack, not the value in r0 that will be overwritten by the function
call.

Finally, add CFI directives describing the save/restore of r2.

With the fix gdb can correctly back trace and navigate up and down the stack:

  Breakpoint 1, 0x00007ffff7f804dc in __kernel_clock_gettime ()
  (gdb) bt
  #0  0x00007ffff7f804dc in __kernel_clock_gettime ()
  #1  0x00007ffff7d8872c in clock_gettime@@GLIBC_2.17 () from /lib64/libc.so.6
  #2  0x0000000100015b60 in gettime ()
  #3  0x000000010000c8bc in print_long_format ()
  #4  0x000000010000d180 in print_current_files ()
  #5  0x00000001000054ac in main ()
  (gdb) up
  #1  0x00007ffff7d8872c in clock_gettime@@GLIBC_2.17 () from /lib64/libc.so.6
  (gdb)
  #2  0x0000000100015b60 in gettime ()
  (gdb)
  #3  0x000000010000c8bc in print_long_format ()
  (gdb)
  #4  0x000000010000d180 in print_current_files ()
  (gdb)
  #5  0x00000001000054ac in main ()
  (gdb)
  Initial frame selected; you cannot go up.
  (gdb) down
  #4  0x000000010000d180 in print_current_files ()
  (gdb)
  #3  0x000000010000c8bc in print_long_format ()
  (gdb)
  #2  0x0000000100015b60 in gettime ()
  (gdb)
  #1  0x00007ffff7d8872c in clock_gettime@@GLIBC_2.17 () from /lib64/libc.so.6
  (gdb)
  #0  0x00007ffff7f804dc in __kernel_clock_gettime ()
  (gdb)

Fixes: ce7d8056e38b ("powerpc/vdso: Prepare for switching VDSO to generic C implementation.")
Cc: stable@vger.kernel.org # v5.11+
Reported-by: Alan Modra <amodra@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Link: https://lore.kernel.org/r/20220502125010.1319370-1-mpe@ellerman.id.au

diff --git a/arch/powerpc/kernel/vdso/gettimeofday.S b/arch/powerpc/kernel/vdso/gettimeofday.S
index eb9c81e1c218526d1ddc0ac76885894ed0fe74ec..0c4ecc8fec5a64d1f56f5ee4cf9dc80760ac24ce 100644 (file)
--- a/arch/powerpc/kernel/vdso/gettimeofday.S
+++ b/arch/powerpc/kernel/vdso/gettimeofday.S
@@ -22,12 +22,15 @@
 .macro cvdso_call funct call_time=0
   .cfi_startproc
        PPC_STLU        r1, -PPC_MIN_STKFRM(r1)
+  .cfi_adjust_cfa_offset PPC_MIN_STKFRM
        mflr            r0
-  .cfi_register lr, r0
        PPC_STLU        r1, -PPC_MIN_STKFRM(r1)
+  .cfi_adjust_cfa_offset PPC_MIN_STKFRM
        PPC_STL         r0, PPC_MIN_STKFRM + PPC_LR_STKOFF(r1)
+  .cfi_rel_offset lr, PPC_MIN_STKFRM + PPC_LR_STKOFF
 #ifdef __powerpc64__
        PPC_STL         r2, PPC_MIN_STKFRM + STK_GOT(r1)
+  .cfi_rel_offset r2, PPC_MIN_STKFRM + STK_GOT
 #endif
        get_datapage    r5
        .ifeq   \call_time
@@ -39,13 +42,15 @@
        PPC_LL          r0, PPC_MIN_STKFRM + PPC_LR_STKOFF(r1)
 #ifdef __powerpc64__
        PPC_LL          r2, PPC_MIN_STKFRM + STK_GOT(r1)
+  .cfi_restore r2
 #endif
        .ifeq   \call_time
        cmpwi           r3, 0
        .endif
        mtlr            r0
-  .cfi_restore lr
        addi            r1, r1, 2 * PPC_MIN_STKFRM
+  .cfi_restore lr
+  .cfi_def_cfa_offset 0
        crclr           so
        .ifeq   \call_time
        beqlr+