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`F_GETPIPE_SZ` returns `EINVAL`, and `F_SETLEASE` returns 65536

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
72/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
c
Domain
operating-systems

Research direction

Start in sys_fcntl in src/syscall/fs.c, where the switch uses case 1024 for F_GETPIPE_SZ, and compare its command numbers with Linux's F_LINUX_SPECIFIC_BASE offsets from the issue. Compile the reproducer and run it on a pipe and a regular file. Done when F_GETPIPE_SZ returns the pipe size, F_SETLEASE on a pipe returns EINVAL, and F_SETLEASE on a file returns 0, matching the Linux row in the table.

Written by the indexing model from the issue text.

Description

bug

sys_fcntl (src/syscall/fs.c) answers F_GETPIPE_SZ under case 1024. Linux counts these commands from F_LINUX_SPECIFIC_BASE, which is 1024: F_SETLEASE is 1024, F_GETLEASE is 1025, F_SETPIPE_SZ is 1031 and F_GETPIPE_SZ is 1032. The real F_GETPIPE_SZ therefore reaches the default arm and returns EINVAL, and F_SETLEASE returns 65536 on any fd, which its caller reads as success.

Reproducer:

#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

int main(void)
{
    int p[2];
    pipe(p);
    int file = open("/tmp/lease-file", O_CREAT | O_RDONLY, 0600);

    errno = 0;
    printf("F_GETPIPE_SZ on a pipe: %d errno=%d\n", fcntl(p[0], F_GETPIPE_SZ),
           errno);
    errno = 0;
    printf("F_SETLEASE on a pipe:   %d errno=%d\n",
           fcntl(p[0], F_SETLEASE, F_RDLCK), errno);
    errno = 0;
    printf("F_SETLEASE on a file:   %d errno=%d\n",
           fcntl(file, F_SETLEASE, F_RDLCK), errno);
    unlink("/tmp/lease-file");
    return 0;
}
F_GETPIPE_SZ on a pipe F_SETLEASE on a pipe F_SETLEASE on a file
elfuse b57f367 -1, EINVAL 65536 65536
Linux 6.12 under qemu 65536 -1, EINVAL 0

F_SETPIPE_SZ is under the right number. It returns its argument and leaves the host pipe as it is, which the comment beside it says.

Dominant language
C
Stars
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Forks
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Avg merge
2d 22h
Merged PRs (30d)
18

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