[PATCH V3 RESEND] drm/i915: Fix "mitigations" parsing if i915 is builtin

[Jisheng Zhang]

I met below error during boot with i915 builtin if pass
"i915.mitigations=off":
[    0.015589] Booting kernel: `off' invalid for parameter `i915.mitigations'

The reason is slab subsystem isn't ready at that time, so kstrdup()
returns NULL. Fix this issue by using stack var instead of kstrdup().

Fixes: 984cadea032b ("drm/i915: Allow the sysadmin to override security mitigations")
Signed-off-by: Jisheng Zhang <Jisheng.Zhang at synaptics.com>
---
Since v2:
 - Use strscpy() per Ville's suggestion

Since v1:
 - Ensure "str" is properly terminated. Thanks Ville for pointing this out

 drivers/gpu/drm/i915/i915_mitigations.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_mitigations.c b/drivers/gpu/drm/i915/i915_mitigations.c
index 84f12598d145..70944764a77e 100644
--- a/drivers/gpu/drm/i915/i915_mitigations.c
+++ b/drivers/gpu/drm/i915/i915_mitigations.c
@@ -29,15 +29,13 @@ bool i915_mitigate_clear_residuals(void)
 static int mitigations_set(const char *val, const struct kernel_param *kp)
 {
 	unsigned long new = ~0UL;
-	char *str, *sep, *tok;
+	char str[64], *sep, *tok;
 	bool first = true;
 	int err = 0;
 
 	BUILD_BUG_ON(ARRAY_SIZE(names) >= BITS_PER_TYPE(mitigations));
 
-	str = kstrdup(val, GFP_KERNEL);
-	if (!str)
-		return -ENOMEM;
+	strscpy(str, val, sizeof(str));
 
 	for (sep = str; (tok = strsep(&sep, ","));) {
 		bool enable = true;
@@ -86,7 +84,6 @@ static int mitigations_set(const char *val, const struct kernel_param *kp)
 			break;
 		}
 	}
-	kfree(str);
 	if (err)
 		return err;
 
-- 
2.31.0