[PATCH v3] ALSA: core: Fix info read handling

[Sreeraj S Kurup]

snd_info_entry_read() may invoke driver read callbacks with a
zero-length read when the requested count is zero.

Some callbacks perform zero-sized allocations, such as
vmalloc(0), which can trigger warnings and cause the read to
fail with -ENOMEM.

Return immediately when count is zero to avoid invoking driver
callbacks unnecessarily.

Also, store the callback result in a separate ssize_t variable
instead of reusing the size_t variable. This makes the signedness
explicit and avoids the unnecessary cast when checking whether
to update the file position.

Update the file position only when the callback returns a
positive number of bytes.

Fixes: d97e1b78239c ("ALSA: info - Check file position validity in common layer")
Signed-off-by: Sreeraj S Kurup <sreekuttan2156239@xxxxxxxxx>
---
v3:
 - Add Fixes trailer for the commit that introduced the zero-length
   read issue.
v2:
 - Handle zero-length reads in snd_info_entry_read() by returning
   early without invoking the driver callback.
 - Clarify the commit message to describe the ssize_t change as a
   signedness cleanup rather than claiming it fixes incorrectly
   reported callback errors.

 sound/core/info.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/sound/core/info.c b/sound/core/info.c
index c4d57d0809ad..42312136f76c 100644
--- a/sound/core/info.c
+++ b/sound/core/info.c
@@ -144,6 +144,7 @@ static ssize_t snd_info_entry_read(struct file *file, char __user *buffer,
 	struct snd_info_private_data *data = file->private_data;
 	struct snd_info_entry *entry = data->entry;
 	size_t size;
+	ssize_t ret;
 	loff_t pos;
 
 	pos = *offset;
@@ -151,13 +152,15 @@ static ssize_t snd_info_entry_read(struct file *file, char __user *buffer,
 		return -EIO;
 	if (pos >= entry->size)
 		return 0;
+	if (!count)
+		return 0;
 	size = entry->size - pos;
 	size = min(count, size);
-	size = entry->c.ops->read(entry, data->file_private_data,
+	ret = entry->c.ops->read(entry, data->file_private_data,
 				  file, buffer, size, pos);
-	if ((ssize_t) size > 0)
-		*offset = pos + size;
-	return size;
+	if (ret > 0)
+		*offset = pos + ret;
+	return ret;
 }
 
 static ssize_t snd_info_entry_write(struct file *file, const char __user *buffer,
-- 
2.55.0