On 03/14/17 09:29, Thomas Garnier wrote:
We want to enforce the address limit by default, not only when
CONFIG_BUG_ON_DATA_CORRUPTION is enabled. I tested this one:
/* Check user-mode state on fast path return. */
movq PER_CPU_VAR(current_task), %rax
btq $63, TASK_addr_limit(%rax)
jnc 1f
#ifdef CONFIG_BUG_ON_DATA_CORRUPTION
call syscall_return_slowpath
jmp return_from_SYSCALL_64
#else
movq $TASK_SIZE_MAX, %rcx
movq %rcx, TASK_addr_limit(%rax)
#endif
1:
I saw that syscall_return_slowpath is supposed to be called not jumped
to. I could just call verify_pre_usermode_state that would be about
the same.
I wanted to comment on that thing: why on earth isn't
verify_pre_usermode_state() an inline? Making it an out-of-line
function adds pointless extra overhead to the C code when we are talking
about a few instructions.
Second, you never do a branch-around to handle an exceptional condition
on the fast path: you jump *out of line* to handle the special
condition; a forward branch is preferred since it is slightly more
likely to be predicted not taken.
Now, I finally had a chance to actually look at the full file (I was
preoccupied yesterday), and am a bit disappointed, to say the least.
First of all, the jump target you need is only a handful of instructions
further down the code path; you need to do *exactly* that is done when
the test of _TIF_ALLWORK_MASK right above is tested! Not only that, but
you already have PER_CPU_VAR(current_task) in %r11 just ready to be
used! This was all in the three instructions immediately prior to the
code you modified...
So, all you'd need would be:
movq $TASK_SIZE_MAX, %rcx
cmpq %rcx, TASK_addr_limit(%r11)
jne 1f
We even get a short jump instruction!
(Using bt saves one more instruction, but see previous caveats about it.)
-hpa