mtd_add_partition() does not reject the special offset value
MTDPART_OFS_RETAIN (-3), which leads to a WARN_ON in
add_mtd_device() when called through the BLKPG ioctl on NAND
devices. The RETAIN value depends on cur_offset being the end of
the previous partition, but in the dynamic partition path
cur_offset equals the offset argument itself, causing undefined
behavior.

Commit 5daa7b21496a ("mtd: prepare partition add and del functions
for ioctl requests") introduced mtd_add_partition() and correctly
rejected MTDPART_OFS_APPEND (-1) and MTDPART_OFS_NXTBLK (-2),
since those special offsets rely on cur_offset tracking the
previous partition's end. However, commit 1a31368bf92e ("mtd: add a flags
for partitions which should just leave smth. after them")
later added MTDPART_OFS_RETAIN (-3) for the static
partition table path without updating mtd_add_partition() to
also reject this value.

With offset=-3 passed via BLKPG, the RETAIN size calculation in
allocate_partition() underflows (parent_size - 0xFFFFFFFFFFFFFFFD
= parent_size + 3). If the underflow result does not appear to
leave enough space, allocate_partition() jumps to out_register via
goto, skipping erasesize initialization. This results in
erasesize=0, which triggers:

WARN_ON((!mtd->erasesize || !master->_erase) &&
		!(mtd->flags & MTD_NO_ERASE))

in add_mtd_device(). If the underflow result appears to leave
enough space, a bogus partition size is calculated, but the
"out of reach" sanity check catches the invalid offset and
creates a disabled empty partition (offset=0, size=0) instead
of returning an error.

Fix this by adding MTDPART_OFS_RETAIN to the rejection list in
mtd_add_partition(), consistent with the existing handling of
APPEND and NXTBLK.

Fixes: 1a31368bf92e ("mtd: add a flags for partitions which should just leave smth. after them")

Signed-off-by: zhouminqiang <zhouminqiang2@huawei.com>
---
 drivers/mtd/mtdpart.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/mtd/mtdpart.c b/drivers/mtd/mtdpart.c
index 795a94e6b482..11cab777a1ac 100644
--- a/drivers/mtd/mtdpart.c
+++ b/drivers/mtd/mtdpart.c
@@ -255,7 +255,8 @@ int mtd_add_partition(struct mtd_info *parent, const char *name,
 
 	/* the direct offset is expected */
 	if (offset == MTDPART_OFS_APPEND ||
-	    offset == MTDPART_OFS_NXTBLK)
+	    offset == MTDPART_OFS_NXTBLK ||
+	    offset == MTDPART_OFS_RETAIN)
 		return -EINVAL;
 
 	if (length == MTDPART_SIZ_FULL)
-- 
2.52.0

On Thu, 16 Jul 2026 21:14:32 +0800, zhouminqiang wrote:
> mtd_add_partition() does not reject the special offset value
> MTDPART_OFS_RETAIN (-3), which leads to a WARN_ON in
> add_mtd_device() when called through the BLKPG ioctl on NAND
> devices. The RETAIN value depends on cur_offset being the end of
> the previous partition, but in the dynamic partition path
> cur_offset equals the offset argument itself, causing undefined
> behavior.
> 
> [...]

Applied to mtd/next, thanks!

[1/1] mtd: part: reject MTDPART_OFS_RETAIN in mtd_add_partition()
      commit: b759d5bb6265419344ee9729fd0dc07ad85719d8

Patche(s) should be available on mtd/linux.git and will be
part of the next PR (provided that no robot complains by then).

Kind regards,
Miquèl

在 2026/7/16 21:14, zhouminqiang 写道:
> mtd_add_partition() does not reject the special offset value
> MTDPART_OFS_RETAIN (-3), which leads to a WARN_ON in
> add_mtd_device() when called through the BLKPG ioctl on NAND
> devices. The RETAIN value depends on cur_offset being the end of
> the previous partition, but in the dynamic partition path
> cur_offset equals the offset argument itself, causing undefined
> behavior.
> 
> Commit 5daa7b21496a ("mtd: prepare partition add and del functions
> for ioctl requests") introduced mtd_add_partition() and correctly
> rejected MTDPART_OFS_APPEND (-1) and MTDPART_OFS_NXTBLK (-2),
> since those special offsets rely on cur_offset tracking the
> previous partition's end. However, commit 1a31368bf92e ("mtd: add a flags
> for partitions which should just leave smth. after them")
> later added MTDPART_OFS_RETAIN (-3) for the static
> partition table path without updating mtd_add_partition() to
> also reject this value.
> 
> With offset=-3 passed via BLKPG, the RETAIN size calculation in
> allocate_partition() underflows (parent_size - 0xFFFFFFFFFFFFFFFD
> = parent_size + 3). If the underflow result does not appear to
> leave enough space, allocate_partition() jumps to out_register via
> goto, skipping erasesize initialization. This results in
> erasesize=0, which triggers:
> 
> WARN_ON((!mtd->erasesize || !master->_erase) &&
> 		!(mtd->flags & MTD_NO_ERASE))
> 
> in add_mtd_device(). If the underflow result appears to leave
> enough space, a bogus partition size is calculated, but the
> "out of reach" sanity check catches the invalid offset and
> creates a disabled empty partition (offset=0, size=0) instead
> of returning an error.
> 
> Fix this by adding MTDPART_OFS_RETAIN to the rejection list in
> mtd_add_partition(), consistent with the existing handling of
> APPEND and NXTBLK.
> 
> Fixes: 1a31368bf92e ("mtd: add a flags for partitions which should just leave smth. after them")
> 
> Signed-off-by: zhouminqiang <zhouminqiang2@huawei.com>
> ---
>   drivers/mtd/mtdpart.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)

Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
> 
> diff --git a/drivers/mtd/mtdpart.c b/drivers/mtd/mtdpart.c
> index 795a94e6b482..11cab777a1ac 100644
> --- a/drivers/mtd/mtdpart.c
> +++ b/drivers/mtd/mtdpart.c
> @@ -255,7 +255,8 @@ int mtd_add_partition(struct mtd_info *parent, const char *name,
>   
>   	/* the direct offset is expected */
>   	if (offset == MTDPART_OFS_APPEND ||
> -	    offset == MTDPART_OFS_NXTBLK)
> +	    offset == MTDPART_OFS_NXTBLK ||
> +	    offset == MTDPART_OFS_RETAIN)
>   		return -EINVAL;
>   
>   	if (length == MTDPART_SIZ_FULL)
>