[LibOS] Ignore O_TRUNC flag for FIFO files

[sahason]

Description of the changes

During enablement of mxnet-model-server in gramine one issue is observed that for FIFO files opening it with O_TRUNC returns EINVAL. In the native O_TRUNC flag for FIFO files is ignored. See man 2 open. This PR also emulates the same behaviour for gramine.

How to test this PR?

Modify this line

gramine/libos/test/regression/mkfifo.c

Line 30 in 702ac42

fd = open(FIFO_PATH, O_NONBLOCK | O_RDONLY);

by adding O_TRUNC flag and replacing O_RDONLY by O_RDWR and test this PR by running regression test.

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This change is 

[dimakuv]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion, not enough approvals from maintainers (3 more required), not enough approvals from different teams (2 more required, approved so far: ) (waiting on @sahason)

a discussion (no related file):
Thanks @sahason for the PR. The idea is correct, but I'll take over this PR and will fix it to be more "Gramine" standard.

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[dimakuv]

Reviewed 3 of 3 files at r2, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions, not enough approvals from maintainers (3 more required), not enough approvals from different teams (2 more required, approved so far: ) (waiting on @sahason)

a discussion (no related file):

Previously, dimakuv (Dmitrii Kuvaiskii) wrote…

Thanks @sahason for the PR. The idea is correct, but I'll take over this PR and will fix it to be more "Gramine" standard.

Done

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a discussion (no related file):
I tested this PR.

• Before this change:

[P1:T1:mkfifo] trace: ---- openat(AT_FDCWD, "tmp/fifo", O_WRONLY|O_TRUNC|0x800, 0000) = -22

• After this change:

[P1:T1:mkfifo] trace: ---- openat(AT_FDCWD, "tmp/fifo", O_WRONLY|O_TRUNC|0x800, 0000) = 0x4

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libos/src/fs/pipe/fs.c line 215 at r2 (raw file):

         * covers most apps seen in the wild (in particular, LTP apps). */
        log_warning("FIFO (named pipe) '%s' cannot be opened in read-write mode in Gramine. "
                    "Treating it as read-only.", dent->name);

This is an unrelated change which I detected when playing with libos/test/regression/mkfifo.c and open(O_RDWR).

Before this change Gramine would output:

warning: FIFO (named pipe) '/' cannot be opened in read-write mode in Gramine. Treating it
 as read-only.

After this change:

warning: FIFO (named pipe) 'fifo' cannot be opened in read-write mode in Gramine. Treating
 it as read-only.

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libos/test/regression/mkfifo.c line 65 at r2 (raw file):

            /* wait until client is ready for read (O_TRUNC flag here is only for testing) */
            errno = 0;
            fd = open(FIFO_PATH, O_NONBLOCK | O_WRONLY | O_TRUNC);

I added O_TRUNC to this server part (opening the write end of the FIFO pipe) purely for testing this corner case.

Same could be added to the client part (as @sahason did in the PR description), but I found it too convoluted for a test -- I don't think applications really do RW on FIFO pipes. Or even if they do, that's technically incorrect.

[dimakuv]

Jenkins, test this please

[kailun-qin]

Reviewed 3 of 3 files at r2, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions, not enough approvals from maintainers (2 more required), not enough approvals from different teams (1 more required, approved so far: Intel) (waiting on @sahason)

[dimakuv]

Reviewable status: all files reviewed, all discussions resolved, not enough approvals from maintainers (1 more required), not enough approvals from different teams (1 more required, approved so far: Intel)

[dimakuv]

Reviewable status: all files reviewed, 1 unresolved discussion, not enough approvals from maintainers (1 more required), not enough approvals from different teams (1 more required, approved so far: Intel) (waiting on @sahason)

a discussion (no related file):
Incidentally, we implemented a similar fix in 1563. So when that one is merged, we can close this PR.

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[dimakuv]

This was fixed as part of #1563 (which will be merged today). Closing.