On Saturday 2021-10-30 18:44, Jeremy Sowden wrote:
>`struct ulogd_unixsock_packet_t` is packed, so taking the address of its
>`struct iphdr payload` member may yield an unaligned pointer value.
That may not be a problem. Dereferencing through a pointer to
a packed struct generates very pessimistic code even when there is no
padding internally:
» cat >>x.cpp <<-EOF
struct ethhdr { unsigned long long a, b; } __attribute__((packed));
unsigned long f(const struct ethhdr *p) { return p->b; }
EOF
» sparc64-linux-gcc -c x.cpp -O2
» objdump -d x.o
_Z1fPK6ethhdr:
save %sp, -144, %sp
stx %i0, [%fp+2039]
ldx [%fp+2039], %i0
ldub [%i0+15], %i2
ldub [%i0+14], %i1
sllx %i1, 8, %i1
or %i1, %i2, %i2
ldub [%i0+13], %i3
...
