On Tue, Feb 6, 2018 at 12:58 PM, Linus Torvalds
[off-list ref] wrote:

On Tue, Feb 6, 2018 at 12:49 PM, Andy Lutomirski [off-list ref] wrote:

quoted

Can you use @cc to make an asm statement that outputs both the masked
array index and the "if" condition?  I can never remember the syntax,
but something like:

Yes. Although I'd actually suggest just using an "asm goto" if we
really want to optimize this. Give the "index_mask_nospec()" a third
argument that is the label to jump to for overflow.

Then you can just decide how to implement it best for any particular
architecture (and compiler limitation).

At that point we're basically just back to the array_ptr() version
that returned a sanitized pointer to an array element.

        call = array_ptr(sys_call_table, nr & __SYSCALL_MASK, NR_syscalls);
        if (likely(call))
                regs->ax = (*call)(
                        regs->di, regs->si, regs->dx,
                        regs->r10, regs->r8, regs->r9);


     e1e:       ba 4d 01 00 00          mov    $0x14d,%edx
     e23:       48 39 d5                cmp    %rdx,%rbp
     e26:       48 19 d2                sbb    %rdx,%rdx
        call = array_ptr(sys_call_table, nr & __SYSCALL_MASK, NR_syscalls);
     e29:       48 21 d5                and    %rdx,%rbp
     e2c:       48 8d 04 ed 00 00 00    lea    0x0(,%rbp,8),%rax
     e33:       00
        if (likely(call))
     e34:       48 21 d0                and    %rdx,%rax
     e37:       74 1e                   je     e57 <do_syscall_64+0x77>
                regs->ax = (*call)(
     e39:       48 8b 4b 38             mov    0x38(%rbx),%rcx
     e3d:       48 8b 53 60             mov    0x60(%rbx),%rdx
     e41:       48 8b 73 68             mov    0x68(%rbx),%rsi
     e45:       48 8b 7b 70             mov    0x70(%rbx),%rdi
     e49:       4c 8b 4b 40             mov    0x40(%rbx),%r9
     e4d:       4c 8b 43 48             mov    0x48(%rbx),%r8
     e51:       ff 10                   callq  *(%rax)
     e53:       48 89 43 50             mov    %rax,0x50(%rbx)
     e57:       65 48 8b 04 25 00 00    mov    %gs:0x0,%rax