Re: [PATCH] powerpc/uaccess: Fix inline assembly for clang build on PPC32

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

Hi!

On Tue, Feb 03, 2026 at 10:19:39PM +0000, David Laight wrote:
> On Tue,  3 Feb 2026 08:30:41 +0100
> "Christophe Leroy (CS GROUP)" <chleroy@kernel.org> wrote:
> 
> > Test robot reports the following error with clang-16.0.6:
> > 
> >    In file included from kernel/rseq.c:75:
> >    include/linux/rseq_entry.h:141:3: error: invalid operand for instruction
> >                    unsafe_get_user(offset, &ucs->post_commit_offset, efault);
> >                    ^
> >    include/linux/uaccess.h:608:2: note: expanded from macro 'unsafe_get_user'
> >            arch_unsafe_get_user(x, ptr, local_label);      \
> >            ^
> >    arch/powerpc/include/asm/uaccess.h:518:2: note: expanded from macro 'arch_unsafe_get_user'
> >            __get_user_size_goto(__gu_val, __gu_addr, sizeof(*(p)), e); \
> >            ^
> >    arch/powerpc/include/asm/uaccess.h:284:2: note: expanded from macro '__get_user_size_goto'
> >            __get_user_size_allowed(x, ptr, size, __gus_retval);    \
> >            ^
> >    arch/powerpc/include/asm/uaccess.h:275:10: note: expanded from macro '__get_user_size_allowed'
> >            case 8: __get_user_asm2(x, (u64 __user *)ptr, retval);  break;  \
> >                    ^
> >    arch/powerpc/include/asm/uaccess.h:258:4: note: expanded from macro '__get_user_asm2'
> >                    "       li %1+1,0\n"                    \
> >                     ^
> >    <inline asm>:7:5: note: instantiated into assembly here
> >            li 31+1,0
> >               ^
> >    1 error generated.
> > 
> > On PPC32, for 64 bits vars a pair of registers is used. Usually the
> > lower register in the pair is the high part and the higher register is
> > the low part. GCC uses r3/r4 ... r11/r12 ... r14/r15 ... r30/r31
> > 
> > In older kernel code inline assembly was using %1 and %1+1 to represent
> > 64 bits values. However here it looks like clang uses r31 as high part,
> > allthough r32 doesn't exist hence the error.
> > 
> > Allthoug %1+1 should work, most places now use %L1 instead of %1+1, so
> > let's do the same here.
> > 
> > With that change, the build doesn't fail anymore and a disassembly shows
> > clang uses r17/r18 and r31/r14 pair when GCC would have used r16/r17 and
> > r30/r31:
> 
> Isn't it all horribly worse than that?
> It only failed because clang picked r31, but if can pick two non-adjacent
> registers might it not pick any pair.
> In which case there could easily be a 64bit get_user() that reads an incorrect
> value and corrupts another register.
> Find one and you might have a privilege escalation.

I don't think LLVM is that broken, it only has problems for some edge
cases.  Yes, I might expect too much.  But without proof to the contrary
let's assume things are okay :-)

And, worrying.  But what can we do against it!  Other than never ever
use LLVM for anything serious, of course.


Segher