On Tue, 3 Feb 2026 08:30:41 +0100
"Christophe Leroy (CS GROUP)" <[email protected]> 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.
David
>
> Disassembly of section .fixup:
>
> 00000000 <.fixup>:
> 0: 38 a0 ff f2 li r5,-14
> 4: 3a 20 00 00 li r17,0
> 8: 3a 40 00 00 li r18,0
> c: 48 00 00 00 b c <.fixup+0xc>
> c: R_PPC_REL24 .text+0xbc
> 10: 38 a0 ff f2 li r5,-14
> 14: 3b e0 00 00 li r31,0
> 18: 39 c0 00 00 li r14,0
> 1c: 48 00 00 00 b 1c <.fixup+0x1c>
> 1c: R_PPC_REL24 .text+0x144
>
> Reported-by: kernel test robot <[email protected]>
> Closes:
> https://lore.kernel.org/oe-kbuild-all/[email protected]/
> Fixes: c20beffeec3c ("powerpc/uaccess: Use flexible addressing with
> __put_user()/__get_user()")
> Signed-off-by: Christophe Leroy (CS GROUP) <[email protected]>
> ---
> I set Fixes: tag to the commit that recently replaced %1+1 by %L1 in the main
> part of the macro as the fix would be uncomplete otherwise but the problem
> has been there since commit 2df5e8bcca53 ("powerpc: merge uaccess.h")
> ---
> arch/powerpc/include/asm/uaccess.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/include/asm/uaccess.h
> b/arch/powerpc/include/asm/uaccess.h
> index ba1d878c3f404..570b3d91e2e40 100644
> --- a/arch/powerpc/include/asm/uaccess.h
> +++ b/arch/powerpc/include/asm/uaccess.h
> @@ -255,7 +255,7 @@ __gus_failed:
> \
> ".section .fixup,\"ax\"\n" \
> "4: li %0,%3\n" \
> " li %1,0\n" \
> - " li %1+1,0\n" \
> + " li %L1,0\n" \
> " b 3b\n" \
> ".previous\n" \
> EX_TABLE(1b, 4b) \
