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

[Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>]

Hi Uli,

This patch is ok. I wonder if libgo is working with PPC32, since we don't have
CFI FDE directives for that on ppc32 code. And do you think it is worth to add
a testcase for it?


On 12-11-2013 19:19, Ulrich Weigand wrote:
> 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