lib/posix-poll: Fix poll() ignoring fd 0

[andreittr]

Description of changes

This change fixes a bug where poll() would ignore fd 0 as if it were a negative fd, which are explicitly allowed as NOP entries.

Prerequisite checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes against relevant architectures and platforms;
• Ran the checkpatch.uk on your commit series before opening this PR;
• Updated relevant documentation.

Base target

• Architecture(s): N/A
• Platform(s): N/A
• Application(s): N/A

Additional configuration

CONFIG_LIBPOSIX_POLL=y

Test snippet (assumes fd:0 is tty stdin):

struct pollfd fds = {
	.fd = 0,
	.events = POLLIN|POLLOUT
};
int r = poll(&fds, 1, 1000);
printf("%d %d %hd\n", r, errno, fds.revents);

On Linux poll immediately returns with POLLOUT set.
On staging, with or without this fix, Unikraft waits until the timeout, because vfscore stdio files never emit events.

Applying #1226 that implements tty files more consistently we can test the above snippet.
(GCC 13 or clang is required, otherwise you also need #1245)

Without fix, Unikraft waits until the timeout, with the fix it immediately returns POLLIN|POLLOUT.

[razvand]

Hi, @andreittr . How does this bug manifest?

[andreittr]

Hi, @andreittr . How does this bug manifest?

@razvand I've updated the description; turns out it needs quite a bit of setup to trigger.

[mariasfiraiala]

Thanks @andreittr, I was able to reproduce the issue with #1226 and gcc 13.2 and the fix works just fine. All good on my side!

Reviewed-by: Maria Sfiraiala maria.sfiraiala@gmail.com

[razvand]

Approved-by: Razvan Deaconescu razvand@unikraft.io