[head tinderbox] failure on arm/arm

[M. Warner Losh]

In message: <4557825E.3070009 at errno.com>
            Sam Leffler <sam at errno.com> writes:
: Ruslan Ermilov wrote:
: > On Sun, Nov 12, 2006 at 01:21:05PM -0500, Alexander Kabaev wrote:
: >> GCC expects 4-byte aligned structured on ARM but does not necessarily
: >> have to. We can change the default at the expense of possible more
: >> inefficient code generated and lost binary compatibility with other ARM
: >> OSes out there. So this is trade off between unclear performance
: >> penalty and an unspecified but certainly sizable number of other
: >> landmines like this lurking on the code.
: >>
: >> We should decide what evil we regard as lesser.
: >>
: > This is the only buildworld problem so far on FreeBSD/ARM, so my
: > feeling is that we can actually benefit from leaving it "as is",
: > as it has a potential of making our code more portable.  Of course
: > if binary compatibility for structs across platforms is an issue,
: > a structure should be "packed", because otherwise the C standard
: > says that "Each non-bit-field member of a structure or union object
: > is aligned in an implementation-defined manner appropriate to its
: > type."
: > 
: > On the other hand, having GCC align "struct foo { char x[11]; }"
: > on a four-byte boundary (other than for backward compatibility)
: > doesn't make too much sense to me.
: > 
: > I don't know GCC rules for alignment of structure members.  For
: > example, if it's guaranteed (in GCC) that offsetof(struct foo, bar)
: > is always 1 for "struct foo { char foo; char bar; }" (without the
: > "packed" attribute) on all platforms and OSes GCC supports?
: > I'd expect the latter to be "4" for FreeBSD/ARM but fortunately
: > it stays "1", i.e., only the structure alignment is affected,
: > and not of structure members (which is POLA but makes the 4 byte
: > for structure alignment questionable).
: 
: This issue appears to have broken if_bridge.  On my avila board I align
: rx packets to be aligned s.t. the ip header lands on a 32-bit boundary.
:  This results in the ethernet header being 2-byte aligned which is ok on
: other platforms.  But the compiler takes this code in bridge_forward:
: 
:         /*
:          * If the interface is learning, and the source
:          * address is valid and not multicast, record
:          * the address.
:          */
:         if ((bif->bif_flags & IFBIF_LEARNING) != 0 &&
:             ETHER_IS_MULTICAST(eh->ether_shost) == 0 &&
:             (eh->ether_shost[0] == 0 &&
:              eh->ether_shost[1] == 0 &&
:              eh->ether_shost[2] == 0 &&
:              eh->ether_shost[3] == 0 &&
:              eh->ether_shost[4] == 0 &&
:              eh->ether_shost[5] == 0) == 0) {
:                 (void) bridge_rtupdate(sc, eh->ether_shost,
:                     src_if, 0, IFBAF_DYNAMIC);
:         }
: 
: and converts the 6 byte compares to a 32-bit and 16-bit compare.  Since
: the data is only 16-bit aligned the 32-bit load faults.

Yea, that's clearly bogus of it.  It does this because it thinks that
eh is going to be 4-byte aligned, which it isn't in this case.  I
think that we may need to change:

/*
 * Structure of a 10Mb/s Ethernet header.
 */
struct	ether_header {
	u_char	ether_dhost[ETHER_ADDR_LEN];
	u_char	ether_shost[ETHER_ADDR_LEN];
	u_short	ether_type;
};

to be

struct	ether_header {
	u_char	ether_dhost[ETHER_ADDR_LEN];
	u_char	ether_shost[ETHER_ADDR_LEN];
	u_short	ether_type;
} __packed;

since that would fit.

There's one caveat that I'd caution people about.  NetBSD had lots of
issues with gcc4 and packed when the struct doesn't need to be packed.
But, I must say, that they do flag these as packed:

/*
 * Ethernet address - 6 octets
 * this is only used by the ethers(3) functions.
 */
struct ether_addr {
        u_int8_t ether_addr_octet[ETHER_ADDR_LEN];
} __attribute__((__packed__));

/*
 * Structure of a 10Mb/s Ethernet header.
 */
struct  ether_header {
        u_int8_t  ether_dhost[ETHER_ADDR_LEN];
        u_int8_t  ether_shost[ETHER_ADDR_LEN];
        u_int16_t ether_type;
} __attribute__((__packed__));

(note: in FreeBSD we have #define in sys/cdefs.h:
#define __packed        __attribute__((__packed__))
)

: So the point is that just because things compile doesn't necessarily
: mean they work.  And worse code that works on many/most other
: architectures may not work.

I've fought this same issue in the boot code (look at boot2 for how I
worked around it).  The arm really really hates unaligned accesses...

Warner