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libreboot-utils: simplify random tmpdir namegen

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generalise it in rand.c because this logic will
be useful for other programs in the future.

Signed-off-by: Leah Rowe <leah@libreboot.org>
This commit is contained in:
Leah Rowe
2026-03-29 09:10:57 +01:00
parent 45edcf33f7
commit c2a70b7de0
6 changed files with 57 additions and 82 deletions
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util/libreboot-utils/lib/mkhtemp.c
+11 -41
View File
@@ -627,6 +627,7 @@ mkhtemp_try_create(int dirfd,
struct stat st_open;
int saved_errno = errno;
int rval = -1;
char *rstr = NULL;
int file_created = 0;
int dir_created = 0;
@@ -636,8 +637,13 @@ mkhtemp_try_create(int dirfd,
if_err(*fd >= 0, EEXIST))
goto err;
if (if_err_sys(mkhtemp_fill_random(p, xc) < 0) ||
if_err_sys(fd_verify_dir_identity(dirfd, st_dir_initial) < 0))
/* TODO: potential infinite loop under entropy failure.
* if attacker has control of rand - TODO: maybe add timeout
*/
memcpy(p, rstr = rchars(xc), xc);
free_and_set_null(&rstr);
if (if_err_sys(fd_verify_dir_identity(dirfd, st_dir_initial) < 0))
goto err;
if (type == MKHTEMP_FILE) {
@@ -765,6 +771,7 @@ mkhtemp_tmpfile_linux(int dirfd,
int tmpfd = -1;
size_t retries;
int linked = 0;
char *rstr = NULL;
if (fd == NULL || st == NULL ||
fname_copy == NULL || p == NULL ||
@@ -785,8 +792,8 @@ mkhtemp_tmpfile_linux(int dirfd,
for (retries = 0; retries < MKHTEMP_RETRY_MAX; retries++) {
if (mkhtemp_fill_random(p, xc) < 0)
goto err;
memcpy(p, rstr = rchars(xc), xc);
free_and_set_null(&rstr);
if (fd_verify_dir_identity(dirfd,
st_dir_initial) < 0)
@@ -832,43 +839,6 @@ err:
}
#endif
/* TODO: potential infinite loop under entropy failure.
* e.g. keeps returning low quality RNG, or atacker
* has control (DoS attack potential).
* possible solution: add a timeout (and abort if
* the timeout is reached)
*/
int
mkhtemp_fill_random(char *p, size_t xc)
{
static const char ch[] =
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
unsigned char scratch[64];
size_t off = 0;
size_t i;
/* clamp rand to prevent modulo bias */
size_t limit = 256 - (256 % (sizeof(ch) - 1));
if (if_err(p == NULL, EFAULT))
return -1;
retry_rand:
rset(scratch, sizeof(scratch));
for (i = 0; i < sizeof(scratch) && off < xc; i++) {
if (scratch[i] < limit)
p[off++] = ch[scratch[i] % (sizeof(ch) - 1)];
}
if (off < xc)
goto retry_rand;
return 0;
}
/* WARNING: **ONCE** per file.
*
* some of these checks will trip up