Re: [PATCH] powerpc: Change SDT argument constraint

[Mark Wielaard <mark@klomp.org>]

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.

Thanks,

Mark