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author | Yann Sionneau <ysionneau@kalray.eu> | 2020-01-28 15:27:47 +0100 |
---|---|---|
committer | Waldemar Brodkorb <wbx@openadk.org> | 2020-01-30 09:26:42 +0100 |
commit | 24681d328c46df64062dc67d0af4ef2d7ca6da0c (patch) | |
tree | fb24d0eb54be421612fbb80562a2e217ddf08663 /libc/sysdeps/linux/aarch64/vfork.S | |
parent | fbe25933d475de36fca739154162e668db2b125f (diff) |
poll: avoid calling select with empty sets which hangs the process
Avoid calling select with empty sets which hangs the process
This makes uClibc-ng act like glibc and musl
Without this fix the test_poll of python3 testsuite hangs forever
Scenario of the issue:
If you call poll with only invalid file descriptors, like in python3
testsuite
(https://github.com/python/cpython/blob/master/Lib/test/test_poll.py#L83)
You will go through uClibc poll emulation code, which is based on
select syscall.
Your first call to select will fail, it will return -1 and errno will be
set to EBADF: https://github.com/wbx-github/uclibc-ng/blob/master/libc/sysdeps/linux/common/poll.c#L120
Then you will go through the for loop which tests individually each file descriptor by calling
select on each one: https://github.com/wbx-github/uclibc-ng/blob/master/libc/sysdeps/linux/common/poll.c#L163
each call will also return -1 with errno being equal to EBADF.
Therefore all pollfd will have the POLLNVAL flag in their respective revents field.
And, the most important, rset/wset/xset will stay empty.
Then the for loop ends, the "continue" makes the while loop run again.
The following select() is run again: https://github.com/wbx-github/uclibc-ng/blob/master/libc/sysdeps/linux/common/poll.c#L120
But this time the sets are empty.
If the poll was called with timeout set to -1, this select will hang forever because there is no timeout
and the sets are empty so no event will ever wake it up.
test program:
int main(void)
{
struct pollfd pfd;
int ret;
int pipe_fds[2];
pipe(pipe_fds);
close(pipe_fds[0]);
close(pipe_fds[1]);
pfd.fd = pipe_fds[0];
pfd.events = POLLIN | POLLOUT | POLLPRI;
pfd.revents = 0;
ret = poll(&pfd, 1, -1);
printf("ret: %d\n", ret);
if (ret < 0)
printf("error: %s", strerror(errno));
else {
puts("revents: ");
if (pfd.revents & POLLERR)
printf(" POLLERR");
if (pfd.revents & POLLHUP)
printf(" POLLHUP");
if (pfd.revents & POLLNVAL)
printf(" POLLNVAL");
puts("");
}
return 0;
}
This hangs on uClibc-ng aarch64 and Kalray's arch (kv3) but does the following on musl and glibc:
"
ret: 1
revents:
POLLNVAL
"
strace output of this program with uClibc *without* the patch applied:
pselect6(4, [3], [3], [3], NULL, NULL) = -1 EBADF (Bad file descriptor)
pselect6(4, [3], [3], [3], {tv_sec=0, tv_nsec=0}, NULL) = -1 EBADF (Bad file descriptor)
pselect6(0, 0x7ffffffb80, 0x7ffffffb68, 0x7ffffffb50, NULL, NULL
(never finishes)
strace output of this program with uClibc *with* the patch applied:
pselect6(4, [3], [3], [3], NULL, NULL) = -1 EBADF (Bad file descriptor)
pselect6(4, [3], [3], [3], {tv_sec=0, tv_nsec=0}, NULL) = -1 EBADF (Bad file descriptor)
write(1, "ret: 1\n", 7ret: 1
) = 7
write(1, "revents: \n", 10revents:
) = 10
write(1, " POLLNVAL\n", 10 POLLNVAL
) = 10
exit_group(0) = ?
+++ exited with 0 +++
Diffstat (limited to 'libc/sysdeps/linux/aarch64/vfork.S')
0 files changed, 0 insertions, 0 deletions