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ext/session: fix session GC fork #22164

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ext/session: fix session GC fork #22164

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ef12ecd
fix: session GC fork
jorgsowa May 27, 2026
f5501ea
fix: lazy-reseed GC random state per worker process
jorgsowa Jun 10, 2026
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1 change: 1 addition & 0 deletions ext/session/php_session.h
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Original file line number Diff line number Diff line change
Expand Up @@ -148,6 +148,7 @@ typedef struct _php_ps_globals {
int module_number;
php_random_status_state_pcgoneseq128xslrr64 random_state;
php_random_algo_with_state random;
bool gc_random_seeded; /* false until first GC check; reseeds after fork */
zend_long gc_probability;
zend_long gc_divisor;
zend_long gc_maxlifetime;
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25 changes: 16 additions & 9 deletions ext/session/session.c
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Original file line number Diff line number Diff line change
Expand Up @@ -392,8 +392,22 @@ static zend_long php_session_gc(bool immediate)

/* GC must be done before reading session data. */
if ((PS(mod_data) || PS(mod_user_implemented))) {
/* Use probability-based GC if not forced and probability is configured */
if (!collect && PS(gc_probability) > 0) {
/* Reseed PS(random) on first use per process. GINIT runs in the
* master process before SAPIs like PHP-FPM fork their workers, so
* all workers would otherwise inherit and replay the same PCG
* sequence. One CSPRNG call here breaks the correlation. */
if (UNEXPECTED(!PS(gc_random_seeded))) {
php_random_uint128_t seed;
if (php_random_bytes_silent(&seed, sizeof(seed)) == FAILURE) {
seed = php_random_uint128_constant(
php_random_generate_fallback_seed(),
php_random_generate_fallback_seed()
);
}
php_random_pcgoneseq128xslrr64_seed128(PS(random).state, seed);
PS(gc_random_seeded) = true;
}
collect = php_random_range(PS(random), 0, PS(gc_divisor) - 1) < PS(gc_probability);
}

Expand Down Expand Up @@ -2891,14 +2905,7 @@ static PHP_GINIT_FUNCTION(ps)
.algo = &php_random_algo_pcgoneseq128xslrr64,
.state = &ps_globals->random_state,
};
php_random_uint128_t seed;
if (php_random_bytes_silent(&seed, sizeof(seed)) == FAILURE) {
seed = php_random_uint128_constant(
php_random_generate_fallback_seed(),
php_random_generate_fallback_seed()
);
}
php_random_pcgoneseq128xslrr64_seed128(ps_globals->random.state, seed);
ps_globals->gc_random_seeded = false;
}

static PHP_MINIT_FUNCTION(session)
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