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authorLasse Collin <lasse.collin@tukaani.org>2013-06-28 23:48:05 +0300
committerLasse Collin <lasse.collin@tukaani.org>2013-06-28 23:48:05 +0300
commit9dc319eabb34a826f4945f91c71620f14a60e9e2 (patch)
tree7f92afc9d6f9031d9dcfded0f8231fd5ee7f16d6 /src
parentxz: Use non-blocking I/O for the input file. (diff)
downloadxz-9dc319eabb34a826f4945f91c71620f14a60e9e2.tar.xz
xz: Use the self-pipe trick to avoid a race condition with signals.
It is possible that a signal to set user_abort arrives right before a blocking system call is made. In this case the call may block until another signal arrives, while the wanted behavior is to make xz clean up and exit as soon as possible. After this commit, the race condition is avoided with the input side which already uses non-blocking I/O. The output side still uses blocking I/O and thus has the race condition.
Diffstat (limited to 'src')
-rw-r--r--src/xz/file_io.c56
-rw-r--r--src/xz/file_io.h8
-rw-r--r--src/xz/signals.c5
3 files changed, 57 insertions, 12 deletions
diff --git a/src/xz/file_io.c b/src/xz/file_io.c
index fd22dc4f..21cdecb0 100644
--- a/src/xz/file_io.c
+++ b/src/xz/file_io.c
@@ -51,6 +51,10 @@ static bool restore_stdin_flags = false;
/// io_open_dest() and io_close_dest() to save and restore the flags.
static int stdout_flags;
static bool restore_stdout_flags = false;
+
+/// Self-pipe used together with the user_abort variable to avoid
+/// race conditions with signal handling.
+static int user_abort_pipe[2];
#endif
@@ -70,6 +74,14 @@ io_init(void)
// If fchown() fails setting the owner, we warn about it only if
// we are root.
warn_fchown = geteuid() == 0;
+
+ if (pipe(user_abort_pipe)
+ || fcntl(user_abort_pipe[0], F_SETFL, O_NONBLOCK)
+ == -1
+ || fcntl(user_abort_pipe[1], F_SETFL, O_NONBLOCK)
+ == -1)
+ message_fatal(_("Error creating a pipe: %s"),
+ strerror(errno));
#endif
#ifdef __DJGPP__
@@ -82,6 +94,17 @@ io_init(void)
}
+#ifndef TUKLIB_DOSLIKE
+extern void
+io_write_to_user_abort_pipe(void)
+{
+ uint8_t b = '\0';
+ (void)write(user_abort_pipe[1], &b, 1);
+ return;
+}
+#endif
+
+
extern void
io_no_sparse(void)
{
@@ -91,11 +114,20 @@ io_no_sparse(void)
#ifndef TUKLIB_DOSLIKE
-/// \brief Waits for input or output to become available
+/// \brief Waits for input or output to become available or for a signal
+///
+/// This uses the self-pipe trick to avoid a race condition that can occur
+/// if a signal is caught after user_abort has been checked but before e.g.
+/// read() has been called. In that situation read() could block unless
+/// non-blocking I/O is used. With non-blocking I/O something like select()
+/// or poll() is needed to avoid a busy-wait loop, and the same race condition
+/// pops up again. There are pselect() (POSIX-1.2001) and ppoll() (not in
+/// POSIX) but neither is portable enough in 2013. The self-pipe trick is
+/// old and very portable.
static bool
io_wait(file_pair *pair, bool is_reading)
{
- struct pollfd pfd[1];
+ struct pollfd pfd[2];
if (is_reading) {
pfd[0].fd = pair->src_fd;
@@ -105,18 +137,17 @@ io_wait(file_pair *pair, bool is_reading)
pfd[0].events = POLLOUT;
}
- while (true) {
- const int ret = poll(pfd, 1, -1);
+ pfd[1].fd = user_abort_pipe[0];
+ pfd[1].events = POLLIN;
- if (ret == -1) {
- if (errno == EINTR) {
- if (user_abort)
- return true;
+ while (true) {
+ const int ret = poll(pfd, 2, -1);
- continue;
- }
+ if (user_abort)
+ return true;
- if (errno == EAGAIN)
+ if (ret == -1) {
+ if (errno == EINTR || errno == EAGAIN)
continue;
message_error(_("%s: poll() failed: %s"),
@@ -125,7 +156,8 @@ io_wait(file_pair *pair, bool is_reading)
strerror(errno));
}
- return false;
+ if (pfd[0].revents != 0)
+ return false;
}
}
#endif
diff --git a/src/xz/file_io.h b/src/xz/file_io.h
index ef639324..2de33792 100644
--- a/src/xz/file_io.h
+++ b/src/xz/file_io.h
@@ -68,6 +68,14 @@ typedef struct {
extern void io_init(void);
+#ifndef TUKLIB_DOSLIKE
+/// \brief Write a byte to user_abort_pipe[1]
+///
+/// This is called from a signal handler.
+extern void io_write_to_user_abort_pipe(void);
+#endif
+
+
/// \brief Disable creation of sparse files when decompressing
extern void io_no_sparse(void);
diff --git a/src/xz/signals.c b/src/xz/signals.c
index de213644..2a1d4eb7 100644
--- a/src/xz/signals.c
+++ b/src/xz/signals.c
@@ -41,6 +41,11 @@ signal_handler(int sig)
{
exit_signal = sig;
user_abort = true;
+
+#ifndef TUKLIB_DOSLIKE
+ io_write_to_user_abort_pipe();
+#endif
+
return;
}