05d290d66b
If a parent and child process open the two ends of a fifo, and the
child immediately exits, the parent may receive a SIGCHLD before its
open() returns. In that case, we need to make sure that open() will
return successfully after the SIGCHLD handler returns, instead of
throwing EINTR or being restarted. Otherwise, the restarted open()
would incorrectly wait for a second partner on the other end.
The following test demonstrates the EINTR that was wrongly thrown from
the parent’s open(). Change .sa_flags = 0 to .sa_flags = SA_RESTART
to see a deadlock instead, in which the restarted open() waits for a
second reader that will never come. (On my systems, this happens
pretty reliably within about 5 to 500 iterations. Others report that
it manages to loop ~forever sometimes; YMMV.)
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define CHECK(x) do if ((x) == -1) {perror(#x); abort();} while(0)
void handler(int signum) {}
int main()
{
struct sigaction act = {.sa_handler = handler, .sa_flags = 0};
CHECK(sigaction(SIGCHLD, &act, NULL));
CHECK(mknod("fifo", S_IFIFO | S_IRWXU, 0));
for (;;) {
int fd;
pid_t pid;
putc('.', stderr);
CHECK(pid = fork());
if (pid == 0) {
CHECK(fd = open("fifo", O_RDONLY));
_exit(0);
}
CHECK(fd = open("fifo", O_WRONLY));
CHECK(close(fd));
CHECK(waitpid(pid, NULL, 0));
}
}
This is what I suspect was causing the Git test suite to fail in
t9010-svn-fe.sh:
http://bugs.debian.org/678852
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
153 lines
3.2 KiB
C
153 lines
3.2 KiB
C
/*
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* linux/fs/fifo.c
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*
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* written by Paul H. Hargrove
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*
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* Fixes:
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* 10-06-1999, AV: fixed OOM handling in fifo_open(), moved
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* initialization there, switched to external
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* allocation of pipe_inode_info.
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*/
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#include <linux/mm.h>
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#include <linux/fs.h>
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#include <linux/sched.h>
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#include <linux/pipe_fs_i.h>
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static int wait_for_partner(struct inode* inode, unsigned int *cnt)
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{
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int cur = *cnt;
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while (cur == *cnt) {
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pipe_wait(inode->i_pipe);
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if (signal_pending(current))
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break;
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}
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return cur == *cnt ? -ERESTARTSYS : 0;
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}
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static void wake_up_partner(struct inode* inode)
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{
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wake_up_interruptible(&inode->i_pipe->wait);
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}
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static int fifo_open(struct inode *inode, struct file *filp)
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{
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struct pipe_inode_info *pipe;
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int ret;
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mutex_lock(&inode->i_mutex);
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pipe = inode->i_pipe;
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if (!pipe) {
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ret = -ENOMEM;
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pipe = alloc_pipe_info(inode);
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if (!pipe)
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goto err_nocleanup;
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inode->i_pipe = pipe;
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}
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filp->f_version = 0;
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/* We can only do regular read/write on fifos */
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filp->f_mode &= (FMODE_READ | FMODE_WRITE);
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switch (filp->f_mode) {
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case FMODE_READ:
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/*
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* O_RDONLY
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* POSIX.1 says that O_NONBLOCK means return with the FIFO
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* opened, even when there is no process writing the FIFO.
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*/
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filp->f_op = &read_pipefifo_fops;
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pipe->r_counter++;
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if (pipe->readers++ == 0)
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wake_up_partner(inode);
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if (!pipe->writers) {
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if ((filp->f_flags & O_NONBLOCK)) {
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/* suppress POLLHUP until we have
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* seen a writer */
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filp->f_version = pipe->w_counter;
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} else {
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if (wait_for_partner(inode, &pipe->w_counter))
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goto err_rd;
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}
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}
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break;
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case FMODE_WRITE:
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/*
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* O_WRONLY
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* POSIX.1 says that O_NONBLOCK means return -1 with
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* errno=ENXIO when there is no process reading the FIFO.
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*/
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ret = -ENXIO;
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if ((filp->f_flags & O_NONBLOCK) && !pipe->readers)
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goto err;
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filp->f_op = &write_pipefifo_fops;
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pipe->w_counter++;
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if (!pipe->writers++)
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wake_up_partner(inode);
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if (!pipe->readers) {
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if (wait_for_partner(inode, &pipe->r_counter))
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goto err_wr;
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}
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break;
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case FMODE_READ | FMODE_WRITE:
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/*
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* O_RDWR
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* POSIX.1 leaves this case "undefined" when O_NONBLOCK is set.
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* This implementation will NEVER block on a O_RDWR open, since
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* the process can at least talk to itself.
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*/
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filp->f_op = &rdwr_pipefifo_fops;
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pipe->readers++;
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pipe->writers++;
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pipe->r_counter++;
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pipe->w_counter++;
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if (pipe->readers == 1 || pipe->writers == 1)
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wake_up_partner(inode);
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break;
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default:
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ret = -EINVAL;
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goto err;
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}
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/* Ok! */
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mutex_unlock(&inode->i_mutex);
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return 0;
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err_rd:
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if (!--pipe->readers)
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wake_up_interruptible(&pipe->wait);
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ret = -ERESTARTSYS;
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goto err;
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err_wr:
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if (!--pipe->writers)
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wake_up_interruptible(&pipe->wait);
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ret = -ERESTARTSYS;
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goto err;
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err:
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if (!pipe->readers && !pipe->writers)
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free_pipe_info(inode);
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err_nocleanup:
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mutex_unlock(&inode->i_mutex);
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return ret;
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}
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/*
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* Dummy default file-operations: the only thing this does
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* is contain the open that then fills in the correct operations
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* depending on the access mode of the file...
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*/
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const struct file_operations def_fifo_fops = {
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.open = fifo_open, /* will set read_ or write_pipefifo_fops */
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.llseek = noop_llseek,
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};
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