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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