[PATCH] mm/mmap: Tighten up cmdline_parse_stack_guard_gap()

[Anshuman Khandual <anshuman.khandual@arm.com>]

Currently kernel command line 'stack_guard_gap=', which does not provide an
explicit pages gap value, still assigns 0 to 'stack_guard_gap', which would
not have been expected. In such cases it should just retain the default gap
value (DEFAULT_STACK_GUARD_GAP). Instead let's assert a positive value for
input gap pages before proceeding any further. While here, this tightens up
cmdline_parse_stack_guard_gap() for other scenarios, where the command line
parameter 'stack_guard_gap' is not successfully handled as expected.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
Depends on the following patch.

https://lore.kernel.org/all/20230828035248.678960-1-anshuman.khandual@arm.com/

 mm/mmap.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 8679750333bb..adaa81d95518 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2122,16 +2122,19 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
 /* enforced gap between the expanding stack and other mappings. */
 unsigned long stack_guard_gap = DEFAULT_STACK_GUARD_GAP;
 
-static int __init cmdline_parse_stack_guard_gap(char *p)
+static int __init cmdline_parse_stack_guard_gap(char *str)
 {
 	unsigned long val;
-	char *endptr;
 
-	val = simple_strtoul(p, &endptr, 10);
-	if (!*endptr)
-		stack_guard_gap = val << PAGE_SHIFT;
+	if (!str)
+		return 0;
 
-	return 1;
+	val = simple_strtoul(str, &str, 10);
+	if (!*str && val) {
+		stack_guard_gap = val << PAGE_SHIFT;
+		return 1;
+	}
+	return 0;
 }
 __setup("stack_guard_gap=", cmdline_parse_stack_guard_gap);
 
-- 
2.30.2