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/*
* Copyright (c) 2026 Denys Vlasenko <vda.linux@googlemail.com>
*
* Licensed under GPLv2, see file LICENSE in this source tree.
*/
/*
* This program is a CGI application. It is intended to rate-limit
* invocations of another, presumably resource-intensive CGI
* which you want to only allow less than N instances at any one time.
*
* Any extra clients who try to run the CGI will get the
* "429 Too Many Requests" HTTP response.
*
* The most efficient way to do so is to use a shebang-style executable file:
* #!/path/to/httpd_ratelimit_cgi /tmp/lockdir 99 /path/to/expensive_cgi
*/
/* Build a-la
i486-linux-uclibc-gcc \
-static -static-libgcc \
-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 \
-Wall -Wshadow -Wwrite-strings -Wundef -Wstrict-prototypes -Werror \
-Wold-style-definition -Wdeclaration-after-statement -Wno-pointer-sign \
-Wmissing-prototypes -Wmissing-declarations \
-Os -fno-builtin-strlen -finline-limit=0 -fomit-frame-pointer \
-ffunction-sections -fdata-sections -fno-guess-branch-probability \
-funsigned-char \
-falign-functions=1 -falign-jumps=1 -falign-labels=1 -falign-loops=1 \
-march=i386 -mpreferred-stack-boundary=2 \
-Wl,-Map -Wl,link.map -Wl,--warn-common -Wl,--sort-common -Wl,--gc-sections \
httpd_ratelimit_cgi.c -o httpd_ratelimit_cgi
*/
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <signal.h>
#include <sys/stat.h> /* mkdir */
#include <limits.h>
static void full_write(int fd, const void *buf, size_t len)
{
ssize_t cc;
while (len) {
cc = write(fd, buf, len);
if (cc < 0)
return;
buf = ((const char *)buf) + cc;
len -= cc;
}
}
static void full_write2(int fd, const char *msg, const char *msg2)
{
full_write(fd, msg, strlen(msg));
full_write(fd, " '", 2);
full_write(fd, msg2, strlen(msg2));
full_write(fd, "'\n", 2);
}
static void write_and_die(int fd, const char *msg)
{
full_write(fd, msg, strlen(msg));
exit(0);
}
static void write_and_die2(int fd, const char *msg, const char *msg2)
{
full_write2(fd, msg, msg2);
exit(0);
}
static void fmt_ul(char *dst, unsigned long n)
{
char buf[sizeof(n)*3 + 2];
char *p;
p = buf + sizeof(buf) - 1;
*p = '\0';
do {
*--p = (n % 10) + '0';
n /= 10;
} while (n);
strcpy(dst, p);
}
static long get_no(const char *s)
{
const char *start = s;
long v = 0;
while (*s >= '0' && *s <= '9')
v = v * 10 + (*s++ - '0');
if (start == s || *s != '\0' /*|| v < 0*/)
return -1;
return v;
}
int main(int argc, char **argv)
{
const char *lock_dir = ".";
unsigned long max_slots;
char *sp;
char *symno;
unsigned slot_num;
pid_t my_pid;
char my_pid_str[sizeof(long)*3];
argv++;
if (!argv[0] || !argv[1])
write_and_die(2, "Usage: ratelimit [LOCKDIR] MAX_PROCS PROG [ARGS]\n");
/* ratelimit "[LOCKDIR] MAX_PROCS PROG" SHEBANG [ARGS] syntax?
* This happens if we are running as shebang file
* of the form "!#/path/to/ratelimit [/tmp/cgit] 10 CGI_BINARY"
* (in this case argv[1] is the shebang's filename) */
sp = strchr(argv[0], ' ');
if (sp) {
*sp++ = '\0';
/* convert to ratelimit "SOME\0THING" SHEBANG [ARGS] form */
/* argv1 ^ */
argv[1] = sp;
sp = strchr(sp, ' ');
if (sp) { /* "THING" also has a space? There is a LOCKDIR! */
*sp++ = '\0';
/* convert to ratelimit "SOME\0THI\0G" SHEBANG [ARGS] form */
/* argv0^ ^argv1 */
lock_dir = argv[0];
argv[0] = argv[1];
argv[1] = sp;
goto get_max;
}
}
max_slots = get_no(argv[0]);
if (max_slots > 9999) {
/* ratelimit LOCKDIR MAX_PROCS PROG [ARGS] */
lock_dir = argv[0];
if (!lock_dir[0])
write_and_die2(2, "Bad LOCKDIR", argv[0]);
argv++;
get_max:
max_slots = get_no(argv[0]);
if (max_slots > 9999)
write_and_die2(2, "Bad MAX_PROCS", argv[0]);
}
argv++; /* points to PROG [ARGS] */
{
char slot_path[strlen(lock_dir) + 16];
symno = stpcpy(stpcpy(slot_path, lock_dir), "/lock.");
my_pid = getpid();
fmt_ul(my_pid_str, my_pid);
/* Ensure lock directory exists (idempotent, ignores errors) */
if (lock_dir[0] != '.' || lock_dir[1]) /* Don't bother with "." */
mkdir(lock_dir, 0755);
/* Starting slot varies per process */
slot_num = my_pid;
/* max_slots = 0 is allowed for testing */
if (max_slots != 0) for (int i = 0; i < max_slots; i++) {
slot_num = (slot_num + 1) % max_slots;
fmt_ul(symno, slot_num);
while (1) {
char buf[32];
ssize_t len;
long old_pid;
/* Try to claim atomically */
if (symlink(my_pid_str, slot_path) == 0)
goto exec;
/* Only handle EEXIST - other errors skip to next slot */
if (errno != EEXIST)
break;
/* Read existing target PID */
len = readlink(slot_path, buf, sizeof(buf) - 1);
if (len < 1) {
/* Broken/empty - clean up and retry */
unlink(slot_path);
continue;
}
buf[len] = '\0';
/* Parse PID */
old_pid = get_no(buf);
if (old_pid <= 0 || old_pid > INT_MAX) {
/* Invalid PID string - clean up and retry */
unlink(slot_path);
continue;
}
/* Check if old process is alive */
if (kill(old_pid, 0) == 0 || errno != ESRCH) {
/* Alive (or unexpected error): slot in use, try next */
break;
}
/* Dead: clean up and retry this slot */
unlink(slot_path);
/* Loop continues to retry symlink() */
}
}
/* No slot available, return 429 */
write_and_die(1, "Status: 429 Too Many Requests\r\n"
"Content-Type: text/plain\r\n"
"Retry-After: 60\r\n"
"Connection: close\r\n\r\n"
"Too many concurrent requests\n"
);
return 0;
}
exec:
execv(argv[0], argv);
full_write2(2, "can't execute", argv[0]);
write_and_die(1, "Status: 500 Internal Server Error\r\n"
"Content-Type: text/plain\r\n\r\n"
"Failed to execute binary\n");
return 1;
}
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