[Tarantool-patches] [PATCH 2/2] fiber: leak slab if unable to bring prots back

[Ilya Kosarev <i.kosarev@tarantool.org>]

In case if we unable to revert guard page back to
read|write we should never use such slab again.

Initially I thought of just put panic here and
exit but it is too destructive. I think better
print an error and continue. If node admin ignore
this message then one moment at future there won't
be slab left for use and creating new fibers get
prohibited.

In future (hopefully near one) we plan to drop
guard pages to prevent VMA fracturing and use
stack marks instead.

Relates to #5211

Co-authored-by: Cyrill Gorcunov <gorcunov@gmail.com>
(backported from commit 8d53fadc0cb15a45ea0cd461d2d5243be51b37e0)
---
 src/fiber.c                  | 11 +++++++++--
 test/unit/fiber_stack.c      | 36 +++++++++++++++++++++++++++++++++---
 test/unit/fiber_stack.result |  5 ++++-
 3 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/src/fiber.c b/src/fiber.c
index 2d83bbc1a..cf6b78c43 100644
--- a/src/fiber.c
+++ b/src/fiber.c
@@ -936,15 +936,22 @@ fiber_stack_destroy(struct fiber *fiber, struct slab_cache *slabc)
 			 * to setup the original protection back in
 			 * background.
 			 *
+			 * For now lets keep such slab referenced and
+			 * leaked: if mprotect failed we must not allow
+			 * to reuse such slab with PROT_NONE'ed page
+			 * inside.
+			 *
 			 * Note that in case if we're called from
 			 * fiber_stack_create() the @a mprotect_flags is
 			 * the same as the slab been created with, so
 			 * calling mprotect for VMA with same flags
 			 * won't fail.
 			 */
-			diag_log();
+			say_syserror("fiber: Can't put guard page to slab. "
+			             "Leak %zu bytes", (size_t) fiber->stack_size);
+		} else {
+			slab_put(slabc, fiber->stack_slab);
 		}
-		slab_put(slabc, fiber->stack_slab);
 	}
 }
 
diff --git a/test/unit/fiber_stack.c b/test/unit/fiber_stack.c
index 103dfaf0d..41abe3764 100644
--- a/test/unit/fiber_stack.c
+++ b/test/unit/fiber_stack.c
@@ -15,11 +15,13 @@ noop_f(va_list ap)
 static int
 main_f(va_list ap)
 {
+	struct slab_cache *slabc = &cord()->slabc;
+	size_t used_before, used_after;
 	struct errinj *inj;
 	struct fiber *fiber;
 
 	header();
-	plan(4);
+	plan(6);
 
 	/*
 	 * Set non-default stack size to prevent reusing of an
@@ -47,8 +49,7 @@ main_f(va_list ap)
 	ok(diag_get() != NULL, "mprotect: diag is armed after error");
 
 	/*
-	 * Check madvise. We can't test the fiber destroy
-	 * path since it is cleaning error.
+	 * Check madvise error on fiber creation.
 	 */
 	diag_clear(diag_get());
 	inj = errinj(ERRINJ_FIBER_MADVISE, ERRINJ_BOOL);
@@ -59,6 +60,35 @@ main_f(va_list ap)
 	ok(fiber != NULL, "madvise: non critical error on madvise hint");
 	ok(diag_get() != NULL, "madvise: diag is armed after error");
 
+	/*
+	 * Check if we leak on fiber destruction.
+	 * We will print an error and result get
+	 * compared by testing engine.
+	 */
+	fiber_attr_delete(fiber_attr);
+	fiber_attr = fiber_attr_new();
+	fiber_attr->flags |= FIBER_CUSTOM_STACK;
+	fiber_attr->stack_size = 64 << 10;
+
+	diag_clear(diag_get());
+
+	used_before = slabc->allocated.stats.used;
+
+	fiber = fiber_new_ex("test_madvise", fiber_attr, noop_f);
+	ok(fiber != NULL, "fiber with custom stack");
+	fiber_set_joinable(fiber, true);
+
+	inj = errinj(ERRINJ_FIBER_MPROTECT, ERRINJ_INT);
+	inj->iparam = PROT_READ | PROT_WRITE;
+
+	fiber_start(fiber);
+	fiber_join(fiber);
+	inj->iparam = -1;
+
+	used_after = slabc->allocated.stats.used;
+	ok(used_after > used_before, "expected leak detected");
+
+	fiber_attr_delete(fiber_attr);
 	footer();
 
 	ev_break(loop(), EVBREAK_ALL);
diff --git a/test/unit/fiber_stack.result b/test/unit/fiber_stack.result
index 43ff74b2f..7cae4e96c 100644
--- a/test/unit/fiber_stack.result
+++ b/test/unit/fiber_stack.result
@@ -1,8 +1,11 @@
 SystemError fiber mprotect failed: Cannot allocate memory
+fiber: Can't put guard page to slab. Leak 57344 bytes: Cannot allocate memory
 	*** main_f ***
-1..4
+1..6
 ok 1 - mprotect: failed to setup fiber guard page
 ok 2 - mprotect: diag is armed after error
 ok 3 - madvise: non critical error on madvise hint
 ok 4 - madvise: diag is armed after error
+ok 5 - fiber with custom stack
+ok 6 - expected leak detected
 	*** main_f: done ***
-- 
2.17.1