Re: Strange EFAULT on mips64el returned by syscall when another thread is forking

[Xi Ruoyao]

On Wed, 2024-01-24 at 14:10 -0800, Linus Torvalds wrote:
> On Wed, 24 Jan 2024 at 13:54, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > 
> > 
> > And I think the "fails with any integer in [1, 8)" is because the MIPS
> > "copy_from_user()" code is likely doing something special for those
> > small copies.
> 
> .Lcopy_bytes_checklen\@: does COPY_BYTE(0) for the first access, which is
> 
> #define COPY_BYTE(N)                    \
>         LOADB(t0, N(src), .Ll_exc\@);   \
>         SUB     len, len, 1;            \
>         beqz    len, .Ldone\@;          \
>         STOREB(t0, N(dst), .Ls_exc_p1\@)
> 
> so yeah, for 'copy_to_user()" (which is what that "read (fd, buf, 7)"
> will do, we have that user space write ("STOREB()") in the branch
> delay slot of the length test.
> 
> So that matches.
> 
> And it only fails when
> 
>  (a) you're unlucky, and that stack buffer
> 
>           char buf[16] = {};
> 
>      happens to be just under the last page that has been accessed, so
> you get a page fault
> 
>  (b) you hit a mmap_sem already being locked, presumably because
> another thread is doing that fork().

So I added a stupid hack:

diff --git a/mm/memory.c b/mm/memory.c
index 7e1f4849463a..e663eb517bbf 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -92,6 +92,12 @@
 #include "internal.h"
 #include "swap.h"
 
+#ifdef __mips__
+#include "asm/branch.h"
+#undef instruction_pointer
+#define instruction_pointer(x) exception_epc(x)
+#endif
+
 #if defined(LAST_CPUPID_NOT_IN_PAGE_FLAGS) && !defined(CONFIG_COMPILE_TEST)
 #warning Unfortunate NUMA and NUMA Balancing config, growing page-frame for last_cpupid.
 #endif

and it indeed "solved" the problem.

> Anyway, I'm pretty sure this is the bug, now some MIPS person just
> needs to fix the MIPS version of "instruction_pointer()" to do what
> "exception_epc()" already does.

Agree.

-- 
Xi Ruoyao <xry111@xxxxxxxxxxx>
School of Aerospace Science and Technology, Xidian University