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[ Upstream commit7bf3a476ce] Miao Wang reported a bug of SO_PEEK_OFF on AF_UNIX SOCK_STREAM socket. The unexpected behaviour is triggered when the peek offset is larger than the recv queue and the thread is unblocked by new data. Let's assume a socket which has "aaaa" in the recv queue and the peek offset is 4. First, unix_stream_read_generic() reads the offset 4 and skips the skb(s) of "aaaa" with the code below: skip = max(sk_peek_offset(sk, flags), 0); /* @skip is 4. */ do { ... while (skip >= unix_skb_len(skb)) { skip -= unix_skb_len(skb); ... skb = skb_peek_next(skb, &sk->sk_receive_queue); if (!skb) goto again; /* @skip is 0. */ } The thread jumps to the 'again' label and goes to sleep since new data has not arrived yet. Later, new data "bbbb" unblocks the thread, and the thread jumps to the 'redo:' label to restart the entire process from the first skb in the recv queue. do { ... redo: ... last = skb = skb_peek(&sk->sk_receive_queue); ... again: if (skb == NULL) { ... timeo = unix_stream_data_wait(sk, timeo, last, last_len, freezable); ... goto redo; /* @skip is 0 !! */ However, the peek offset is not reset in the path. If the buffer size is 8, recv() will return "aaaabbbb" without skipping any data, and the final offset will be 12 (the original offset 4 + peeked skbs' length 8). After sleeping in unix_stream_read_generic(), we have to fetch the peek offset again. Let's move the redo label before mutex_lock(&u->iolock). Fixes:9f389e3567("af_unix: return data from multiple SKBs on recv() with MSG_PEEK flag") Reported-by: Miao Wang <shankerwangmiao@gmail.com> Closes: https://lore.kernel.org/netdev/3B969F90-F51F-4B9D-AB1A-994D9A54D460@gmail.com/ Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com> Link: https://patch.msgid.link/20251117174740.3684604-2-kuniyu@google.com Signed-off-by: Jakub Kicinski <kuba@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.