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rand: fix modulo bias in rmalloc

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Signed-off-by: Leah Rowe <leah@libreboot.org>
This commit is contained in:
Leah Rowe
2026-03-26 08:56:15 +00:00
parent dbc99be9a0
commit cf16d07df9
1 changed files with 8 additions and 2 deletions
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util/libreboot-utils/lib/rand.c
+8 -2
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@@ -81,7 +81,7 @@ win_lottery(char **buf) /* are u lucky? */
char *s1 = rmalloc(&size); char *s1 = rmalloc(&size);
char *s2 = rmalloc(&size); char *s2 = rmalloc(&size);
if (scmp(s1, s2, BUFSIZ + 2, &rval) >= 0 && if (scmp(s1, s2, BUFSIZ + 1, &rval) >= 0 &&
rval == 0) rval == 0)
rval = 1; /* winner! */ rval = 1; /* winner! */
else else
@@ -98,10 +98,16 @@ win_lottery(char **buf) /* are u lucky? */
void * void *
rmalloc(size_t *rval) rmalloc(size_t *rval)
{ {
/* clamp rand to prevent modulo bias */
size_t limit = SIZE_MAX - (SIZE_MAX % BUFSIZ);
if (if_err(rval == NULL, EFAULT)) if (if_err(rval == NULL, EFAULT))
return NULL; return NULL;
rset(rval, sizeof(*rval)); do {
rset(rval, sizeof(*rval));
} while (*rval >= limit);
return mkrstr(*rval %= BUFSIZ); return mkrstr(*rval %= BUFSIZ);
} }