Re: [PATCH] powerpc: Change SDT argument constraint

[Segher Boessenkool <segher@kernel.crashing.org>]

On Thu, Oct 05, 2017 at 12:16:23PM +0200, Mark Wielaard wrote:
> On Thu, 2017-10-05 at 14:39 +0530, Sandipan Das wrote:
> > With the 'o' memory constraint, any memory operand which
> > has an offsettable address is allowed. However, for some
> > architectures such as powerpc, this allows operands like
> > the ones shown below in the readelf output from Fedora 26
> > to be generated.
> > 
> >   $ readelf -n /lib64/libc.so.6 | grep memory_mallopt_mmap_max -A2
> > -B2
> >     stapsdt              0x0000006c	NT_STAPSDT (SystemTap
> > probe descriptors)
> >       Provider: libc
> >       Name: memory_mallopt_mmap_max
> >       Location: 0x00000000000a0274, Base: 0x00000000001ccb90,
> > Semaphore: 0x0000000000000000
> >       Arguments: -4@9 -4@.LANCHOR0+44@toc@l(8) -4@.LANCHOR0+52@toc@l(
> > 7)
> > 
> > The second and third argument shown above are both having
> > operands which are pointers to static data anchors. Since
> > these static anchors are not included in the symbol table,
> > they cannot be resolved from the binary itself. So, such
> > arguments cannot be read via their corresponding markers.
> > 
> > Using the 'Z' memory constraint instead solves this issue
> > as it will only allow a memory operand that is an indexed
> > or indirect from a register.
> > 
> > So, for powerpc, we set STAP_SDT_ARG_CONSTRAINT to 'nZr'
> > but keep it as 'nor' for all other architectures.
> 
> I haven't tested it, but just reading the machine constraints page
> https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html it looks
> correct to me.

I wonder why only PowerPC is hitting this problem though?


Segher