[PATCH] usb: gadget: f_fs: Fix -Wformat-truncation in proxy endpoint names

[PATCH] usb: gadget: f_fs: Fix -Wformat-truncation in proxy endpoint names

[Neill Kapron]

When using the functionfs rw_proxy feature, proxy endpoint names are
generated by appending "_rw" to the base endpoint name. The base
endpoint name is copied via `snprintf` into an 8-byte buffer
(`comp->name`). On some architectures (e.g., s390), the compiler flags
a `-Wformat-truncation` warning because `%s_rw` could potentially
overflow if the source string is unexpectedly long.

Fix this by specifying a maximum field width of 4 characters
when formatting the endpoint name string. This guarantees that at most
4 characters plus the 3 characters "_rw" (and null terminator) will be
written, fitting within the 8-byte limit.

Note that this is purely to address the compiler static analysis. In
practice, `FFS_MAX_EPS_COUNT` is limited to 31, meaning the longest
possible base endpoint name is "ep30", and therefore the resulting
proxy name, "ep30_rw", is exactly 7 characters.

Fixes: 0f0ea552cd21 ("usb: gadget: f_fs: Introduce rw_proxy file descriptors")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202607111052.PRRGX3Jq-lkp@intel.com/
Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Neill Kapron <nkapron@google.com>
---
 drivers/usb/gadget/function/f_fs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 073c4cbd90fb..3f0b445aba19 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -2441,7 +2441,7 @@ static int ffs_epfiles_create(struct ffs_data *ffs)
 			mutex_init(&comp->mutex);
 			mutex_init(&comp->dmabufs_mutex);
 			INIT_LIST_HEAD(&comp->dmabufs);
-			snprintf(comp->name, sizeof(comp->name), "%s_rw",
+			snprintf(comp->name, sizeof(comp->name), "%.4s_rw",
 				 epfiles[i].name);
 			err = ffs_sb_create_file(ffs->sb, comp->name,
 						 comp, &ffs_epfile_operations);
-- 
2.55.0.229.g6434b31f56-goog