This program always prints 4096 and hangs before the patch, and always prints 8192 and exits successfully after:

int main() { int pipefd[2]; for (int i = 0; i < 1025; i++) if (pipe(pipefd) == -1) return 1; size_t bufsz = fcntl(pipefd[1], F_GETPIPE_SZ); printf("%zd\n", bufsz); char *buf = calloc(bufsz, 1); write(pipefd[1], buf, bufsz); read(pipefd[0], buf, bufsz-1); write(pipefd[1], buf, 1); }

Note that you may need to increase your RLIMIT_NOFILE before running the program.

Fixes: 759c01142a ("pipe: limit the per-user amount of pages allocated in pipes") Cc: stable@vger.kernel.org

Link: https://lore.kernel.org/lkml/1628086770.5rn8p04n6j.none@localhost/ Link: https://lore.kernel.org/lkml/1628127094.lxxn016tj7.none@localhost/

Signed-off-by: Alex Xu (Hello71) alex_y_xu@yahoo.ca --- fs/pipe.c | 19 +++++++++++++++++-- 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c index 9ef4231cce61..8e6ef62aeb1c 100644 --- a/fs/pipe.c +++ b/fs/pipe.c @@ -31,6 +31,21 @@

#include "internal.h"

+/* + * New pipe buffers will be restricted to this size while the user is exceeding + * their pipe buffer quota. The general pipe use case needs at least two + * buffers: one for data yet to be read, and one for new data. If this is less + * than two, then a write to a non-empty pipe may block even if the pipe is not + * full. This can occur with GNU make jobserver or similar uses of pipes as + * semaphores: multiple processes may be waiting to write tokens back to the + * pipe before reading tokens: https://lore.kernel.org/lkml/1628086770.5rn8p04n6j.none@localhost/. + * + * Users can reduce their pipe buffers with F_SETPIPE_SZ below this at their + * own risk, namely: pipe writes to non-full pipes may block until the pipe is + * emptied. + */ +#define PIPE_MIN_DEF_BUFFERS 2 + /* * The max size that a non-root user is allowed to grow the pipe. Can * be set by root in /proc/sys/fs/pipe-max-size @@ -781,8 +796,8 @@ struct pipe_inode_info *alloc_pipe_info(void) user_bufs = account_pipe_buffers(user, 0, pipe_bufs);

if (too_many_pipe_buffers_soft(user_bufs) && pipe_is_unprivileged_user()) { - user_bufs = account_pipe_buffers(user, pipe_bufs, 1); - pipe_bufs = 1; + user_bufs = account_pipe_buffers(user, pipe_bufs, PIPE_MIN_DEF_BUFFERS); + pipe_bufs = PIPE_MIN_DEF_BUFFERS; }

if (too_many_pipe_buffers_hard(user_bufs) && pipe_is_unprivileged_user())