mips: terminate the FDE before the return trampoline in makecontext

  In makecontext the FDE needs to be terminated before the return
trampoline otherwise backtrace called within a context created by
makecontext yields infinite backtrace.

This bug has been present for a long time, stdlib/tst-makecontext did
not fail until recent commit e535ce25. Tested on mips-linux-gnu and
mips64el-linux-gnuabi64 and mips-linux-gnu, no regression.

This fixes stdlib/tst-makecontext on MIPS.

Changelog:
	[BZ #19792]
	* sysdeps/unix/sysv/linux/mips/makecontext.S (__makecontext):
	Terminate FDE before return label.
---
 ChangeLog                                  | 6 ++++++
 sysdeps/unix/sysv/linux/mips/makecontext.S | 7 +++++++
 2 files changed, 13 insertions(+)
  

Aurelien Jarno <aurelien@aurel32.net> writes:

> 	[BZ #19792]
> 	* sysdeps/unix/sysv/linux/mips/makecontext.S (__makecontext):
> 	Terminate FDE before return label.

Ok.

Andreas.
  

On Wed, 9 Mar 2016, Aurelien Jarno wrote:

> diff --git a/sysdeps/unix/sysv/linux/mips/makecontext.S b/sysdeps/unix/sysv/linux/mips/makecontext.S
> index 66600c7..3196554 100644
> --- a/sysdeps/unix/sysv/linux/mips/makecontext.S
> +++ b/sysdeps/unix/sysv/linux/mips/makecontext.S
> @@ -153,6 +153,11 @@ NESTED (__makecontext, FRAMESZ, ra)
>  #endif
>  	jr	ra
>  
> +	/* We need to terminate the FDE to stop unwinding if backtrace was
> +	   called within a context created by makecontext.  */
> +	cfi_endproc
> +	nop
> +
>  99:

 What's this NOP needed for here?

  Maciej
  

On 2016-03-22 21:01, Maciej W. Rozycki wrote:
> On Wed, 9 Mar 2016, Aurelien Jarno wrote:
> 
> > diff --git a/sysdeps/unix/sysv/linux/mips/makecontext.S b/sysdeps/unix/sysv/linux/mips/makecontext.S
> > index 66600c7..3196554 100644
> > --- a/sysdeps/unix/sysv/linux/mips/makecontext.S
> > +++ b/sysdeps/unix/sysv/linux/mips/makecontext.S
> > @@ -153,6 +153,11 @@ NESTED (__makecontext, FRAMESZ, ra)
> >  #endif
> >  	jr	ra
> >  
> > +	/* We need to terminate the FDE to stop unwinding if backtrace was
> > +	   called within a context created by makecontext.  */
> > +	cfi_endproc
> > +	nop
> > +
> >  99:
> 
>  What's this NOP needed for here?

We have to separate these to blocks because the unwinder uses ra-1 to
find the FDE.
  

On Wed, 23 Mar 2016, Aurelien Jarno wrote:

> >  What's this NOP needed for here?
> 
> We have to separate these to blocks because the unwinder uses ra-1 to
> find the FDE.

 Ah indeed, I remember now, thanks!

 For the record: I found that bloody piece when trying to make GDB 
interpret DWARF records correctly for compressed code.  The choice to use 
PC-1 in the unwinder arbitrarily across all architectures broke DWARF 
record interpretation in the MIPS port of GDB as the calculation flips the 
ISA bit.  Since it's been like that from the beginning and consequently 
encoded across binaries out there handling GDB had to be adjusted to 
painfully recover the ISA bit from the symbol table as DWARF records are 
interpreted (while the DWARF records should have sufficed by themselves).

 If I had designed this part of the unwinder, I'd have made the adjustment 
target-specific and specifically PC-2 for the MIPS target, i.e. the 
minimum instruction size in the ISA (a rule which should actually work 
universally I believe).

 FWIW,

  Maciej