Assertion `size >= page_size + 1 * page_size' failed.

[Changochen]

Description

The following code:

<?php
$callback = function () {};
ini_set("fiber.stack_size", "");
$fiber = new Fiber($callback);
$fiber->start();

Resulted in this output:

php_asan/Zend/zend_fibers.c:205: zend_fiber_stack *zend_fiber_stack_allocate(size_t): Assertion `size >= page_size + 1 * page_size' failed.

git commit: 84af629

Build config:

./configure --disable-all --enable-address-sanitizer --disable-phpdbg --disable-cgi --with-pic --enable-debug-assertions
make -j

PHP Version

PHP 8.3.0-dev

Operating System

No response

[nielsdos]

I confirm this happens on 8.1 and higher.

The size in zend_fiber_stack_allocate becomes zero. So I guess instead of asserting, we should error out or print a warning and set size to the minimum (page_size + ZEND_FIBER_GUARD_PAGES * page_size).

EDIT: Given that other stack-related issues in that function throw an exception, I guess this should throw as well?
EDIT 2: if an error exception is appropriate, the following patch is maybe good? (applyable using git apply on branch PHP-8.1)

diff --git a/Zend/zend_fibers.c b/Zend/zend_fibers.c
index 1fec85528f..543da605bd 100644
--- a/Zend/zend_fibers.c
+++ b/Zend/zend_fibers.c
@@ -174,8 +174,12 @@ static zend_fiber_stack *zend_fiber_stack_allocate(size_t size)
 {
 	void *pointer;
 	const size_t page_size = zend_fiber_get_page_size();
+	const size_t minimum_stack_size = page_size + ZEND_FIBER_GUARD_PAGES * page_size;
 
-	ZEND_ASSERT(size >= page_size + ZEND_FIBER_GUARD_PAGES * page_size);
+	if (size < minimum_stack_size) {
+		zend_throw_exception_ex(NULL, 0, "Fiber stack size is too small, it needs to be at least %zu bytes", minimum_stack_size);
+		return NULL;
+	}
 
 	const size_t stack_size = (size + page_size - 1) / page_size * page_size;
 	const size_t alloc_size = stack_size + ZEND_FIBER_GUARD_PAGES * page_size;

If it look sensible to the maintainers I'll make a PR

[trowski]

This looks reasonable, please do make a PR. Thank you!