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seedrng: explain why we need locking and fsync'ing

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Also, do not test for locking errors: on Linux, they do not happen.

function                                             old     new   delta
.rodata                                           104900  104878     -22
seedrng_main                                        1022     994     -28

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2022-05-01 01:50:44 +02:00
parent 0bca489b24
commit d5bd2e57a7
1 changed files with 18 additions and 8 deletions
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26
util-linux/seedrng.c
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@ -190,9 +190,17 @@ int seedrng_main(int argc UNUSED_PARAM, char *argv[])
if (mkdir(seed_dir, 0700) < 0 && errno != EEXIST) if (mkdir(seed_dir, 0700) < 0 && errno != EEXIST)
bb_perror_msg_and_die("can't create directory '%s'", seed_dir); bb_perror_msg_and_die("can't create directory '%s'", seed_dir);
dfd = xopen(seed_dir, O_DIRECTORY | O_RDONLY); dfd = xopen(seed_dir, O_DIRECTORY | O_RDONLY);
if (flock(dfd, LOCK_EX) < 0)
bb_perror_msg_and_die("can't lock seed directory");
xfchdir(dfd); xfchdir(dfd);
/* Concurrent runs of this tool might feed the same data to RNG twice.
* Avoid concurrent runs by taking a blocking lock on the directory.
* Not checking for errors. Looking at manpage,
* ENOLCK "The kernel ran out of memory for allocating lock records"
* seems to be the only one which is likely - and if that happens,
* machine is OOMing (much worse problem than inability to lock...).
* Also, typically configured Linux machines do not fail GFP_KERNEL
* allocations (they trigger memory reclaim instead).
*/
flock(dfd, LOCK_EX); /* would block while another copy runs */
sha256_begin(&hash); sha256_begin(&hash);
sha256_hash(&hash, "SeedRNG v1 Old+New Prefix", 25); sha256_hash(&hash, "SeedRNG v1 Old+New Prefix", 25);
@ -204,7 +212,7 @@ int seedrng_main(int argc UNUSED_PARAM, char *argv[])
for (int i = 1; i < 3; ++i) { for (int i = 1; i < 3; ++i) {
seed_from_file_if_exists(i == 1 ? NON_CREDITABLE_SEED_NAME : CREDITABLE_SEED_NAME, seed_from_file_if_exists(i == 1 ? NON_CREDITABLE_SEED_NAME : CREDITABLE_SEED_NAME,
dfd, dfd,
i == 1 ? false : !skip_credit, /* credit? */ i == 1 ? false : !skip_credit,
&hash); &hash);
} }
@ -218,11 +226,13 @@ int seedrng_main(int argc UNUSED_PARAM, char *argv[])
(unsigned)new_seed_len * 8, new_seed_creditable ? "" : "non-"); (unsigned)new_seed_len * 8, new_seed_creditable ? "" : "non-");
fd = xopen3(NON_CREDITABLE_SEED_NAME, O_WRONLY | O_CREAT | O_TRUNC, 0400); fd = xopen3(NON_CREDITABLE_SEED_NAME, O_WRONLY | O_CREAT | O_TRUNC, 0400);
xwrite(fd, new_seed, new_seed_len); xwrite(fd, new_seed, new_seed_len);
if (fsync(fd) < 0) { if (new_seed_creditable) {
bb_perror_msg("can't%s seed", " write"); /* More paranoia when we create a file which we believe contains
return (1 << 4); * genuine entropy: make sure disk is not full, quota was't esceeded, etc:
} */
if (new_seed_creditable) if (fsync(fd) < 0)
bb_perror_msg_and_die("can't write '%s'", NON_CREDITABLE_SEED_NAME);
xrename(NON_CREDITABLE_SEED_NAME, CREDITABLE_SEED_NAME); xrename(NON_CREDITABLE_SEED_NAME, CREDITABLE_SEED_NAME);
}
return EXIT_SUCCESS; return EXIT_SUCCESS;
} }