linux-ext4 - [Bug 218830] lseek on closed file does not trigger an error and affect other files

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Message-ID: <bug-218830-13602-43d0TVLX5P@https.bugzilla.kernel.org/>
Date: Mon, 13 May 2024 13:24:07 +0000
From: bugzilla-daemon@...nel.org
To: linux-ext4@...r.kernel.org
Subject: [Bug 218830] lseek on closed file does not trigger an error and
 affect other files

https://bugzilla.kernel.org/show_bug.cgi?id=218830

Theodore Tso (tytso@....edu) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |tytso@....edu
         Resolution|---                         |INVALID

--- Comment #1 from Theodore Tso (tytso@....edu) ---
This is a test/programming bug.   If you change reproduce.c so that it prints
fd_a  and fd_b, you'll see that they have the same value.   So the reason why
lseek didn't fail is because fd_a has the same integer value as fd_b --- and so
lseek didn't fail and affected the current position of fd_b.

This is a documented feature of the Linux/Unix/Posix interface.   File
descriptors are small integers, and if you close a file descriptor, it releases
that integer ---- think of it as an index in an array, i.e., struct file
*fd_array[MAX_FDS].   When you call open, it will find the first NULL pointer
in fd_array, and installs a pointer to the struct file, and returns that
integer as the file descriptor.

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