On Thu, 2025-04-03 at 12:19 -0700, Nathan Chancellor wrote:
>
> Thanks, I applied that change, which shows a slightly different crash
> message now:
Pretty sure it's all just a bug in my inline assembly, and clang
allocates registers differently:
#define ___backtrack_faulted(_faulted) \
asm volatile ( \
"mov $0, %0\n" \
"movq $__get_kernel_nofault_faulted_%=,%1\n" \
"jmp _end_%=\n" \
"__get_kernel_nofault_faulted_%=:\n" \
"mov $1, %0;" \
"_end_%=:" \
: "=r" (_faulted), \
"=m" (current->thread.segv_continue) :: \
)
It _looks_ as though both %0 and %1 are output only, but clang compiles
it to:
51: 48 83 fb 08 cmp $0x8,%rbx
55: 72 44 jb 9b <_end_0+0x2a>
57: 48 8b 01 mov (%rcx),%rax
// start inline assembly ---vvv--- //
5a: b8 00 00 00 00 mov $0x0,%eax
5f: 48 c7 80 90 07 00 00 movq $0x0,0x790(%rax) // crash
66: 00 00 00 00
66: R_X86_64_32S .text+0x6c
6a: eb 05 jmp 71 <_end_0>
000000000000006c <__get_kernel_nofault_faulted_0>:
6c: b8 01 00 00 00 mov $0x1,%eax
// end inline assembly ---^^^--- //
0000000000000071 <_end_0>:
71: 85 c0 test %eax,%eax
73: 75 56 jne cb <_end_1+0x10>
which clearly cannot work? I must be missing something. Switching the
first two instructions fixes it, of course, but right now I can't see
what I forgot in terms of constraints to make the compiler not do that.
Probably trivial to someone more familiar with inline assembly.
Modifying the _faulted to be +r instead of =r also fixes it.
johannes
