[PATCH] PowerPC64: Fix incorrect CFI in *context routines

["Ulrich Weigand" <uweigand at de dot ibm dot com>]

Hello,

the context established by "makecontext" has a link register pointing
back to an error path within the makecontext routine.  This is currently
covered by the CFI FDE for makecontext itself, which is simply wrong
for the stack frame *inside* the context.  When trying to unwind (e.g.
doing a backtrace) in a routine inside a context created by makecontext,
this can lead to uninitialized stack slots being accessed, causing the
unwinder to crash in the worst case.

Similarly, during parts of the "setcontext" routine, when the stack
pointer has already been switched to point to the new context, the
address range is still covered by the CFI FDE for setcontext.  When
trying to unwind in that situation (e.g. backtrace from an async
signal handler for profiling), it is again possible that the unwinder
crashes.

Theses are all problems in existing code, but the changes in stack
frame layout appear to make the "worst case" much more likely in
the ELFv2 ABI context.  This causes regressions e.g. in the libgo
testsuite on ELFv2.

This patch fixes this by ending the makecontext/setcontext FDEs
before those problematic parts of the assembler, similar to what
is already done on other platforms.   This fixes the libgo
regression on ELFv2.

Tested on powerpc64-linux and powerpc64le-linux.

OK for mainline?

Bye,
Ulrich


ChangeLog:

2013-11-12  Ulrich Weigand  <Ulrich.Weigand@de.ibm.com>

	* sysdeps/unix/sysv/linux/powerpc/powerpc64/makecontext.S
	(__makecontext): Fix incorrect CFI when backtracing out of
	context created via makecontext.
	* sysdeps/unix/sysv/linux/powerpc/powerpc64/secontext.S
	(__setcontext): Fix incorrect CFI during switch to new context.
	(__novec_setcontext): Likewise.

Index: glibc/sysdeps/unix/sysv/linux/powerpc/powerpc64/makecontext.S
===================================================================
--- glibc.orig/sysdeps/unix/sysv/linux/powerpc/powerpc64/makecontext.S
+++ glibc/sysdeps/unix/sysv/linux/powerpc/powerpc64/makecontext.S
@@ -129,6 +129,10 @@ L(noparms):
      the cpu link stack used to predict blr return addresses.  */
   bcl	20,31,L(gotexitcodeaddr);
 
+  /* End FDE now, because while executing on the context's stack
+     the unwind info would be wrong otherwise.  */
+  cfi_endproc
+
 	/* This is the helper code which gets called if a function which
 	   is registered with 'makecontext' returns.  In this case we
 	   have to install the context listed in the uc_link element of
@@ -157,6 +161,11 @@ L(do_exit):
 #endif
 	b    L(do_exit)
 
+  /* Re-establish FDE for the rest of the actual makecontext routine.  */
+  cfi_startproc
+  cfi_offset (lr, FRAME_LR_SAVE)
+  cfi_adjust_cfa_offset (128)
+
   /* The address of the exit code is in the link register.  Store the lr
      in the ucontext as LNK so the target function will return to our
      exit code.  */
Index: glibc/sysdeps/unix/sysv/linux/powerpc/powerpc64/setcontext.S
===================================================================
--- glibc.orig/sysdeps/unix/sysv/linux/powerpc/powerpc64/setcontext.S
+++ glibc/sysdeps/unix/sysv/linux/powerpc/powerpc64/setcontext.S
@@ -129,6 +129,10 @@ ENTRY(__novec_setcontext)
   lfd  fp1,(SIGCONTEXT_FP_REGS+(PT_R1*8))(r31)
   lfd  fp0,(SIGCONTEXT_FP_REGS+(PT_R0*8))(r31)
 
+  /* End FDE now, because the unwind info would be wrong while
+     we're reloading registers to switch to the new context.  */
+  cfi_endproc
+
   ld   r0,(SIGCONTEXT_GP_REGS+(PT_LNK*8))(r31)
   ld   r1,(SIGCONTEXT_GP_REGS+(PT_R1*8))(r31)
   mtlr r0
@@ -177,6 +181,11 @@ ENTRY(__novec_setcontext)
   ld   r31,(SIGCONTEXT_GP_REGS+(PT_R31*8))(r31)
   bctr
 
+  /* Re-establish FDE for the rest of the actual setcontext routine.  */
+  cfi_startproc
+  cfi_offset (lr, FRAME_LR_SAVE)
+  cfi_adjust_cfa_offset (128)
+
 L(nv_error_exit):
   ld   r0,128+FRAME_LR_SAVE(r1)
   addi r1,r1,128
@@ -403,6 +412,10 @@ L(has_no_vec):
   lfd  fp1,(SIGCONTEXT_FP_REGS+(PT_R1*8))(r31)
   lfd  fp0,(SIGCONTEXT_FP_REGS+(PT_R0*8))(r31)
 
+  /* End FDE now, because the unwind info would be wrong while
+     we're reloading registers to switch to the new context.  */
+  cfi_endproc
+
   ld   r0,(SIGCONTEXT_GP_REGS+(PT_LNK*8))(r31)
   ld   r1,(SIGCONTEXT_GP_REGS+(PT_R1*8))(r31)
   mtlr r0
@@ -451,6 +464,11 @@ L(has_no_vec):
   ld   r31,(SIGCONTEXT_GP_REGS+(PT_R31*8))(r31)
   bctr
 
+  /* Re-establish FDE for the rest of the actual setcontext routine.  */
+  cfi_startproc
+  cfi_offset (lr, FRAME_LR_SAVE)
+  cfi_adjust_cfa_offset (128)
+
 L(error_exit):
   ld   r0,128+FRAME_LR_SAVE(r1)
   addi r1,r1,128
-- 
  Dr. Ulrich Weigand
  GNU/Linux compilers and toolchain
  Ulrich.Weigand@de.ibm.com