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    SubjectRe: [PATCH v8 3/8] seccomp: add system call filtering using BPF
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    On Thu, Feb 16, 2012 at 2:06 PM, H. Peter Anvin <hpa@zytor.com> wrote:
    > On 02/16/2012 12:02 PM, Will Drewry wrote:
    >> +
    >> +/* Format of the data the BPF program executes over. */
    >> +struct seccomp_data {
    >> +     int nr;
    >> +     __u32 __reserved[3];
    >> +     struct {
    >> +             __u32   lo;
    >> +             __u32   hi;
    >> +     } instruction_pointer;
    >> +     __u32 lo32[6];
    >> +     __u32 hi32[6];
    >> +};
    >>
    >
    > This seems more than a bit odd, no?
    >
    >        -hpa

    I agree :) BPF being a 32-bit creature introduced some edge cases. I
    has started with a
    union { u32 args32[6]; u64 args64[6]; }

    This was somewhat derailed by CONFIG_COMPAT behavior where
    syscall_get_arguments always writes to argument of register width --
    not bad, just irritating (since a copy isn't strictly necessary nor
    actually done in the patch). Also, Indan pointed out that while BPF
    programs expect constants in the machine-local endian layout, any
    consumers would need to change how they accessed the arguments across
    big/little endian machines since a load of the low-order bits would
    vary.

    In a second pass, I attempted to resolve this like aio_abi.h:
    union {
    struct {
    u32 ENDIAN_SWAP(lo32, hi32);
    };
    u64 arg64;
    } args[6];
    It wasn't clear that this actually made matters better (though it did
    mean syscall_get_arguments() could write directly to arg64). Using
    offsetof() in the user program would be fine, but any offsets set
    another way would be invalid. At that point, I moved to Indan's
    proposal to stabilize low order and high order offsets -- what is in
    the patch series. Now a BPF program can reliably index into the low
    bits of an argument and into the high bits without endianness changing
    the filter program structure.

    I don't feel strongly about any given data layout, and this one seems
    to balance the 32-bit-ness of BPF and the impact that has on
    endianness. I'm happy to hear alternatives that might be more
    aesthetically pleasing :)

    cheers!
    will
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