[PATCH v2] drm/amd/ras: Fix type size of remainder argument

[Kees Cook <kees@kernel.org>]

Forcing an int to be dereferenced at uint64_t for div64_u64_rem() runs
the risk of endian confusion and stack overflowing writes. Seen while
preparing to enable -Warray-bounds globally:

In file included from ../arch/x86/include/asm/processor.h:35,
                 from ../include/linux/sched.h:13,
                 from ../include/linux/ratelimit.h:6,
                 from ../include/linux/dev_printk.h:16,
                 from ../drivers/gpu/drm/amd/amdgpu/../ras/ras_mgr/ras_sys.h:29,
                 from ../drivers/gpu/drm/amd/amdgpu/../ras/rascore/ras.h:27,
                 from ../drivers/gpu/drm/amd/amdgpu/../ras/rascore/ras_core.c:24:
In function 'div64_u64_rem',
    inlined from 'ras_core_convert_timestamp_to_time' at ../drivers/gpu/drm/amd/amdgpu/../ras/rascore/ras_core.c:72:9:
../include/linux/math64.h:56:20: error: array subscript 'u64 {aka long long unsigned int}[0]' is partly outside array bounds of 'int[1]' [-Werror=array-bounds=]
   56 |         *remainder = dividend % divisor;
      |         ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
../drivers/gpu/drm/amd/amdgpu/../ras/rascore/ras_core.c: In function 'ras_core_convert_timestamp_to_time':
../drivers/gpu/drm/amd/amdgpu/../ras/rascore/ras_core.c:70:19: note: object 'remaining_seconds' of size 4
   70 |         int days, remaining_seconds;
      |                   ^~~~~~~~~~~~~~~~~

Use a 64-bit type for the remainder calculation, but leave
remaining_seconds as 32-bit to avoid 64-bit division later. The value of
remainder will always be less than seconds_per_day, so there's no
truncation risk.

Fixes: ace232eff50e ("drm/amdgpu: Add ras module files into amdgpu")
Signed-off-by: Kees Cook <kees@kernel.org>
---
 v2: use temp u64 to avoid 64-bit division later
 v1: https://lore.kernel.org/lkml/20260225024716.work.043-kees@kernel.org/
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: YiPeng Chai <YiPeng.Chai@amd.com>
Cc: Tao Zhou <tao.zhou1@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Cc: <amd-gfx@lists.freedesktop.org>
Cc: <dri-devel@lists.freedesktop.org>
---
 drivers/gpu/drm/amd/ras/rascore/ras_core.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/amd/ras/rascore/ras_core.c b/drivers/gpu/drm/amd/ras/rascore/ras_core.c
index 01122b55c98a..02bbee64a5bd 100644
--- a/drivers/gpu/drm/amd/ras/rascore/ras_core.c
+++ b/drivers/gpu/drm/amd/ras/rascore/ras_core.c
@@ -62,14 +62,16 @@ int ras_core_convert_timestamp_to_time(struct ras_core_context *ras_core,
 			uint64_t timestamp, struct ras_time *tm)
 {
 	int days_in_month[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
-	uint64_t month = 0, day = 0, hour = 0, minute = 0, second = 0;
+	uint64_t month = 0, day = 0, hour = 0, minute = 0, second = 0, remainder;
 	uint32_t year = 0;
 	int seconds_per_day = 24 * 60 * 60;
 	int seconds_per_hour = 60 * 60;
 	int seconds_per_minute = 60;
 	int days, remaining_seconds;
 
-	days = div64_u64_rem(timestamp, seconds_per_day, (uint64_t *)&remaining_seconds);
+	days = div64_u64_rem(timestamp, seconds_per_day, &remainder);
+	/* remainder will always be less than seconds_per_day. */
+	remaining_seconds = remainder;
 
 	/* utc_timestamp follows the Unix epoch */
 	year = 1970;
-- 
2.34.1