Re: ptrace compile failure with gcc-8.2 on 32-bit powerpc

[Michael Ellerman <mpe@ellerman.id.au>]

Meelis Roos <mroos@linux.ee> writes:
> After upgrading my distro compiler to gcc-8.2, Linux fails to compile on 
> 32-bit powerpc (tested with 4.17, 4.18 and v4.18-7873-gf91e654474d4).

Yeah I noticed this just yesterday.

>   CC      arch/powerpc/kernel/ptrace.o
> In file included from ./include/linux/bitmap.h:9,
>                  from ./include/linux/cpumask.h:12,
>                  from ./include/linux/rcupdate.h:44,
>                  from ./include/linux/rculist.h:11,
>                  from ./include/linux/pid.h:5,
>                  from ./include/linux/sched.h:14,
>                  from arch/powerpc/kernel/ptrace.c:19:
> In function ‘memcpy’,
>     inlined from ‘user_regset_copyin’ at ./include/linux/regset.h:295:4,
>     inlined from ‘vr_set’ at arch/powerpc/kernel/ptrace.c:619:9:
> ./include/linux/string.h:345:9: error: ‘__builtin_memcpy’ offset [-527, -529] is out of the bounds [0, 16] of object ‘vrsave’ with type ‘union <anonymous>’ [-Werror=array-bounds]
>   return __builtin_memcpy(p, q, size);
>          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> arch/powerpc/kernel/ptrace.c: In function ‘vr_set’:
> arch/powerpc/kernel/ptrace.c:614:5: note: ‘vrsave’ declared here
>    } vrsave;
>      ^~~~~~

I couldn't actually work out how GCC has decided this is definitely
happening, possibly it just thinks it _could_ happen. I think it's
wrong, but admittedly the code is not easy to follow.

The patch below should fix the build error, but I haven't had time to
test it actually works at runtime:

diff --git a/arch/powerpc/kernel/ptrace.c b/arch/powerpc/kernel/ptrace.c
index 9667666eb18e..607273067d3f 100644
--- a/arch/powerpc/kernel/ptrace.c
+++ b/arch/powerpc/kernel/ptrace.c
@@ -570,7 +570,8 @@ static int vr_get(struct task_struct *target, const struct user_regset *regset,
 		vrsave.word = target->thread.vrsave;
 
 		ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, &vrsave,
-					  33 * sizeof(vector128), -1);
+					  33 * sizeof(vector128),
+					  34 * sizeof(vector128));
 	}
 
 	return ret;
@@ -617,7 +618,8 @@ static int vr_set(struct task_struct *target, const struct user_regset *regset,
 		vrsave.word = target->thread.vrsave;
 
 		ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &vrsave,
-					 33 * sizeof(vector128), -1);
+					 33 * sizeof(vector128),
+					 34 * sizeof(vector128));
 		if (!ret)
 			target->thread.vrsave = vrsave.word;
 	}


cheers