linux-kernel - [PATCH] drm/xe: circumvent bogus stringop-overflow warning

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Date: Wed,  3 Jan 2024 12:48:02 +0100
From: Arnd Bergmann <arnd@...nel.org>
To: Lucas De Marchi <lucas.demarchi@...el.com>,
	Oded Gabbay <ogabbay@...nel.org>,
	Thomas Hellström <thomas.hellstrom@...ux.intel.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>,
	Daniel Vetter <daniel@...ll.ch>,
	Matthew Brost <matthew.brost@...el.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc: Arnd Bergmann <arnd@...db.de>,
	Matt Roper <matthew.d.roper@...el.com>,
	Brian Welty <brian.welty@...el.com>,
	intel-xe@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] drm/xe: circumvent bogus stringop-overflow warning

From: Arnd Bergmann <arnd@...db.de>

gcc-13 warns about an array overflow that it sees but that is
prevented by the "asid % NUM_PF_QUEUE" calculation:

drivers/gpu/drm/xe/xe_gt_pagefault.c: In function 'xe_guc_pagefault_handler':
include/linux/fortify-string.h:57:33: error: writing 16 bytes into a region of size 0 [-Werror=stringop-overflow=]
include/linux/fortify-string.h:689:26: note: in expansion of macro '__fortify_memcpy_chk'
  689 | #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                  \
      |                          ^~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/xe/xe_gt_pagefault.c:341:17: note: in expansion of macro 'memcpy'
  341 |                 memcpy(pf_queue->data + pf_queue->tail, msg, len * sizeof(u32));
      |                 ^~~~~~
drivers/gpu/drm/xe/xe_gt_types.h:102:25: note: at offset [1144, 265324] into destination object 'tile' of size 8

I found that rewriting the assignment using pointer addition rather than the
equivalent array index calculation prevents the warning, so use that instead.

I sent a bug report against gcc for the false positive warning.

Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113214
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 drivers/gpu/drm/xe/xe_gt_pagefault.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/xe/xe_gt_pagefault.c b/drivers/gpu/drm/xe/xe_gt_pagefault.c
index 59a70d2e0a7a..78dc08cc2bfe 100644
--- a/drivers/gpu/drm/xe/xe_gt_pagefault.c
+++ b/drivers/gpu/drm/xe/xe_gt_pagefault.c
@@ -332,7 +332,7 @@ int xe_guc_pagefault_handler(struct xe_guc *guc, u32 *msg, u32 len)
 		return -EPROTO;
 
 	asid = FIELD_GET(PFD_ASID, msg[1]);
-	pf_queue = &gt->usm.pf_queue[asid % NUM_PF_QUEUE];
+	pf_queue = gt->usm.pf_queue + (asid % NUM_PF_QUEUE);
 
 	spin_lock_irqsave(&pf_queue->lock, flags);
 	full = pf_queue_full(pf_queue);
-- 
2.39.2