Re: [PATCH v8 3/8] seccomp: add system call filtering using BPF

[Will Drewry <wad@chromium.org>]

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