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Fix infinite loop on startup if ulimit too low

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Fun fact: rlim_t is an unsigned long long on all platforms.

Continually subtracting from a rlim_t makes it get smaller
and smaller until it wraps, then you're up to 2^64-1.

This was causing an infinite loop on Redis startup if
your ulimit was extremely (almost comically) low.

The case of (f > oldlimit) would never be met in a case like:

    f = 150
    while (f > 20) f -= 128

Since f is unsigned, it can't go negative and would
take on values of:

    Iteration 1: 150 - 128 => 22
    Iteration 2:  22 - 128 => 18446744073709551510
    Iterations 3-∞: ...

To catch the wraparound, we use the previous value of f
stored in limit.rlimit_cur.  If we subtract from f and
get a larger number than the value it had previously,
we print an error and exit since we don't have enough
file descriptors to help the user at this point.

Thanks to @bs3g for the inspiration to fix this problem.
Patches existed from @bs3g at antirez#1227, but I needed to repair a few other
parts of Redis simultaneously, so I didn't get a chance to use them.
This commit is contained in:
Matt Stancliff 2014-03-22 19:29:28 -04:00
parent 4a25983f8f
commit 90b844212d
1 changed files with 11 additions and 0 deletions
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11
src/redis.c
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@ -1519,6 +1519,17 @@ void adjustOpenFilesLimit(void) {
limit.rlim_max = f;
if (setrlimit(RLIMIT_NOFILE,&limit) != -1) break;
f -= REDIS_EVENTLOOP_FDSET_INCR;
if (f > limit.rlim_cur) {
/* Instead of getting smaller, f just got bigger.
* That means it wrapped around its unsigned floor
* and is now closer to 2^64. We can't help anymore. */
redisLog(REDIS_WARNING,"Failed to set max file limit. "
"You requested maxclients of %d "
"but your 'ulimit -n' is set to %llu. "
"Please increase your 'ulimit -n' to at least %llu.",
server.maxclients, oldlimit, maxfiles);
exit(1);
}
}
if (f < oldlimit) f = oldlimit;
if (f != maxfiles) {