Gert Doering <g...@greenie.muc.de> writes:
> Hi,
>
> On Sat, Jun 10, 2017 at 06:16:13PM +0200, Jeremie Courreges-Anglas wrote:
>> Loaded symbols for /usr/libexec/ld.so
>> #0 write_tun_header (tt=0x1ddd4e1900, buf=0x1d336921c5 "E", len=84) at
>> tun.c:1661
>> 1661 if (iph->ip_v == 6)
>> (gdb)
>>
>> buf points to an odd address.
>
> That's a single-byte access...? Nothing to be aligned here.
>
> On FreeBSD, ip_v is defined as such:
>
> * Structure of an internet header, naked of options.
> */
> struct ip {
> #if BYTE_ORDER == LITTLE_ENDIAN
> u_char ip_hl:4, /* header length */
> ip_v:4; /* version */
> #endif
> #if BYTE_ORDER == BIG_ENDIAN
> u_char ip_v:4, /* version */
> ip_hl:4; /* header length */
> #endif
> ...
>
> ... so whatever it is, it's part of a single uchar at the very
> beginning of that header... strange.
OpenBSD:
struct ip {
#if _BYTE_ORDER == _LITTLE_ENDIAN
u_int ip_hl:4, /* header length */
ip_v:4; /* version */
#endif
#if _BYTE_ORDER == _BIG_ENDIAN
u_int ip_v:4, /* version */
ip_hl:4; /* header length */
#endif
...
u_char -> u_int. Dunno if it matters to the compiler, I can take a look
later.
> OTOH, the compiler might assume 16bit-alignment here and use a 16bit-
> access to get the first word, then bit-shift (someone who understands
> sparc64 assembly around to check?) - in this case, the following
> change should help:
>
>
> old:
> if (iph->ip_v == 6)
>
> new:
> if (OPENVPN_IPH_GET_VER(*buf) == 6)
>
> this is an existing macro which takes an int argument, shifting and
> masking the right bits, and since *buf is "uint8_t *buf", this should
> both be correct and alignment-ignorant.
See my proposed patch, also in this thread. I merely copy/pasted the
NetBSD code.
> It might be less efficient than accessing the bit field - but then, it's
> the same work to do, so hopefully the compiler is smart enough to compile
> it to the same instructions.
master (32 bits access):
0x0000000000081844 <write_tun_header+100>: ld [ %i1 ], %g1
0x0000000000081848 <write_tun_header+104>: sethi %hi(0xf0000000), %g2
0x000000000008184c <write_tun_header+108>: sethi %hi(0x60000000), %g3
0x0000000000081850 <write_tun_header+112>: and %g1, %g2, %g1
0x0000000000081854 <write_tun_header+116>: cmp %g1, %g3
patched:
0x0000000000081844 <write_tun_header+100>: ldub [ %i1 ], %g1
0x0000000000081848 <write_tun_header+104>: srl %g1, 4, %g1
0x000000000008184c <write_tun_header+108>: cmp %g1, 6
--
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