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Fix unaligned memory accesses on Alpha and possible other platforms.

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By using a pointer to struct dos_partition, we implicitly tell the
compiler that the pointer is 4-bytes aligned, even though we know
that's not the case. The fact that we only dereference the pointer
to access a byte-wide field (field dp_ptyp) is not a guarantee that
the compiler will in fact use a byte-wide load. On some platforms
it's more efficient to use long word or quad word loads and use
bit-shifting and bit-masking to get the intended byte. On those
platforms an misaligned load will be the result.
The fix is to use byte-wide pointer arithmetic based on sizeof() and
offsetof() to avoid invalid casts which avoids that the compiler
makes invalid assumptions.

Backtrace provided by: wilko@
MFC after: 1 week
This commit is contained in:
Marcel Moolenaar 2006-06-04 20:26:13 +00:00
parent 49ddabdfce
commit ae04949bff
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=159238
1 changed files with 4 additions and 3 deletions
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7
sys/geom/geom_gpt.c
View File

@ -358,8 +358,8 @@ g_gpt_ctl_remove(struct gctl_req *req, const char *flags, struct g_geom *gp,
static int
g_gpt_has_pmbr(struct g_consumer *cp, int *error)
{
struct dos_partition *part;
char *buf;
uint8_t *typ;
int i, pmbr;
uint16_t magic;
@ -373,9 +373,10 @@ g_gpt_has_pmbr(struct g_consumer *cp, int *error)
if (magic != DOSMAGIC)
goto out;
part = (struct dos_partition *)(uintptr_t)(buf + DOSPARTOFF);
for (i = 0; i < 4; i++) {
if (part[i].dp_typ != 0 && part[i].dp_typ != DOSPTYP_PMBR)
typ = buf + DOSPARTOFF + i * sizeof(struct dos_partition) +
offsetof(struct dos_partition, dp_typ);
if (*typ != 0 && *typ != DOSPTYP_PMBR)
goto out;
}