Re: getcpu() returns EFAULT when called via the vdso

[Mike Frysinger <vapier@gentoo.org>]

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On Monday 07 May 2012 13:58:53 Luck, Tony wrote:
> the disassembly of the sched_getcpu() code at runtime looks like:
>    0x2000000000203940 <+0>:     [MMI]       alloc r32=ar.pfs,9,1,0
>    0x2000000000203941 <+1>:                 adds r14=8,r13
>    0x2000000000203942 <+2>:                 mov r33=r12;;
>    0x2000000000203950 <+16>:    [MMI]       ld8 r14=[r14]
>    0x2000000000203951 <+17>:                nop.m 0x0
>    0x2000000000203952 <+18>:                mov r15=1304
>    0x2000000000203960 <+32>:    [MII]       mov r35=r0  <<<<<<<<<<<<<<<<<<<
>    0x2000000000203961 <+33>:                mov r34=r0;;<<<<<<<<<<<<<<<<<<<
>    0x2000000000203962 <+34>:                mov b7=r14;;
>    0x2000000000203970 <+48>:    [MIB]       nop.m 0x0
>    0x2000000000203971 <+49>:                nop.i 0x0
>    0x2000000000203972 <+50>:                br.call.sptk.many b6=b7;;
> 
> When the "br.call" is executed, we flip to the new register
> frame and r34/r35 in the sched_getcpu() frame become r32/r33
> in the new frame.

i think only one register is rotated because we called alloc with locals=1.  
stepping through with gdb shows that -- on the first insn in 
__kernel_syscall_via_epc, r32 is now the local variable on the stack, r33 and 
r34 are 0.

> So you get -EFAULT because the VDSO tries to dereference a NULL
> pointer for each of the *cpu and *node arguments.

the kernel doesn't care if cpu/node are NULL:
SYSCALL_DEFINE3(getcpu, unsigned __user *, cpup, unsigned __user *, nodep,
        struct getcpu_cache __user *, unused)
{
    int err = 0;
    int cpu = raw_smp_processor_id();
    if (cpup)
        err |= put_user(cpu, cpup);
    if (nodep)
        err |= put_user(cpu_to_node(cpu), nodep);
    return err ? -EFAULT : 0;
}
-mike

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