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

[Will Drewry]

On Thu, Feb 16, 2012 at 3:17 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> On 02/16/2012 12:25 PM, Will Drewry wrote:
>>
>>
>> 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).  Usings
>>
>> 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 :)
>>
>
> I would have to say I think native endian is probably the sane thing still,
> out of several bad alternatives.  Certainly splitting the high and low
> halves of arguments is insane.

I'll push the bits around and see how well it plays out in sample/test
code.  Right now, the patch never even populates the data itself - it
just returns four bytes at the requested offset on-demand, so
kernel-side it's pretty simple to do it whatever way seems the least
hideous for the ABI.

> The other thing that you really need in addition to system call number is
> ABI identifier, since a syscall number may mean different things for
> different entry points.  For example, on x86-64 system call number 4 is
> write() if called via int $0x80 but stat() if called via syscall64. This is
> a local property of the system call, not a global per process.

Looks like Markus just replied to this part.  I can certainly populate
a compat bit if the current approach is overconstrained, but I much
prefer to avoid making every user of seccomp need to know about the
subtleties of the calling conventions.

thanks!
will
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