Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 15 Apr 2014 10:46:29 -0700 | Subject | Re: [PATCH] seccomp: fix populating a0-a5 syscall args in 32-bit x86 BPF |
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On Mon, Apr 14, 2014 at 11:31 PM, Alexei Starovoitov <ast@plumgrid.com> wrote: > On Mon, Apr 14, 2014 at 1:28 PM, Andy Lutomirski <luto@amacapital.net> wrote: >> On Mon, Apr 14, 2014 at 1:24 PM, David Miller <davem@davemloft.net> wrote: >>> From: Andy Lutomirski <luto@amacapital.net> >>> Date: Mon, 14 Apr 2014 13:13:45 -0700 >>> >>>> I think this description is wrong. (unsigned long *) &sd->args[1] is >>>> the right location, at least on 32-bit little-endian architectures. >>> >>> It absolutely is not. >> >> Huh? It's a pointer to the right address, but the type is wrong. >> >> The changelog says "on 32-bit x86 (or any other 32bit arch), it would >> result in storing a0-a5 at wrong offsets in args[] member". Unless >> I'm mistaken, this is incorrect: a0-a5 are are the correct offsets, >> but they are stored with the wrong type, so the other bits in there >> are garbage. > > agree. your above description is more correct than the log. > We were focusing on the bug itself and the log came a bit misleading > as a result of multiple iterations back and forth between me and Daniel. > > also the log says: > "gcc is clever and optimizes the copy away in other cases, e.g. x86_64" > since we actually checked assembler, so the fix doesn't pessimize > 64-bit architectures :) > This function is in critical path for seccomp, so performance definitely > matters.
Yeah, I'm not entirely sure what I was thinking when I wrote that part. The new code should actually be much better than the old code for weird architectures like ia-64.
For reference, ia-64 uses the unwinder (!) to look up arguments, so the fewer times it gets invoked, the better.
--Andy
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