Fix read length handling

[Shaoen-Lin]

Function control_read() uses strlen(str) as the source length, but replaces it with the user supplied length when the user buffer is larger. This makes copy_to_user() read past the end of the static string.

The callback also ignores the file offset, so repeated reads do not reach EOF as expected.

Use simple_read_from_buffer() to limit the copy to the requested count and update the offset consistently.

---

Summary by cubic

Fix control_read to cap read length and honor file offsets by using simple_read_from_buffer, preventing reads past the static string and making repeated reads reach EOF.

^{Written for commit 7bf0aba. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}

[jserv]

Check https://cbea.ms/git-commit/ carefully and enforce the rules.

[Shaoen-Lin]

I've modified my commit message.

test case to reproduce:

This test case verifies that reading /dev/vcamctl reaches EOF. Before
the fix, control_read() did not update the file offset, so readers such
as cat kept reading the same string until timeout.

sudo insmod ./vcam.ko
sudo timeout 2 cat /dev/vcamctl
echo $?
sudo rmmod vcam

output before fix:

124

output after fix:

Virtual V4L2 compatible camera device
0

[jserv]

Thank @Shaoen-Lin for contributing!