ffs_vnops: Simplify extattr access

[cem]

Summary

As suggested in r167010, use the structure type and macros to access and
modify UFS2 extended attributes.

PR: 216127
Reported by: dewayne at heuristicsystems.com.au
Sponsored by: Dell EMC Isilon

[kib]

This is fine, but I have one suggestion.

In each place which casts char * into the structure, verify the alignment. This might be not strictly needed right now, since from what I see the pointers are malloced, but might help if the code starts mutating in future. I am not sure whether the checks should be asserts or silently return error.

[cem]

In D9225#190692, @kib wrote:

This is fine, but I have one suggestion.

In each place which casts char * into the structure, verify the alignment. This might be not strictly needed right now, since from what I see the pointers are malloced, but might help if the code starts mutating in future. I am not sure whether the checks should be asserts or silently return error.

Sure, that seems like a good idea. I wasn't too concerned about it because the code takes care to keep struct extattrs (and contents) 8-byte aligned -- and I don't think we require greater alignment outside of e.g. SSE/AVX values.

[cem]

Add correct alignment asserts when casting u_char * to struct extattr *.

[kib]

sys/ufs/ffs/ffs_vnops.c

1110–1112

I have some doubts that the expression is the correct C. In particular, the logical ops with the pointers are not defined, AFAIR. Did you compiled the code with INVARIANTS enabled ?

[cem]

Correct alignment checks by casting through uintptr_t.