[Bug libc/27559] New: fstat(AT_FDCWD) succeeds (it shouldn't) and returns information for the current directory

[Bug libc/27559] New: fstat(AT_FDCWD) succeeds (it shouldn't) and returns information for the current directory

[cptpcrd.git at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=27559

            Bug ID: 27559
           Summary: fstat(AT_FDCWD) succeeds (it shouldn't) and returns
                    information for the current directory
           Product: glibc
           Version: 2.33
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: cptpcrd.git at gmail dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

This can be easily reproduced in Python:

```
>>> import os
>>> os.fstat(-100)  # AT_FDCWD is -100; this should fail with EBADF
```

Or in C:

```
#include <stdio.h>
#include <sys/stat.h>
#include <fcntl.h>

int main() {
    struct stat st;

    // This call should fail with EBADF (bad file descriptor)
    if(fstat(AT_FDCWD, &st) < 0) {
        perror(NULL);
        return 1;
    }

    return 0;
}
```

This seems to have appeared in glibc 2.33.

I strongly suspect that this regression was introduced in one of these commits:
https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=4d97cc8cf3da925fd06fc37d4daebafce3247719
or
https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=30f1c7439489bf756a45e349d69be1826e0c9bd8
(I'm not familiar with glibc's source layout, so I'm not sure which fstat.c
actually gets compiled on Linux).

musl libc had a similar issue:
https://git.etalabs.net/cgit/musl/commit/?id=eeff60608c7aa65806c7c9f0ebddcf2520684ffa
The glibc fix would probably be similar.

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[Bug libc/27559] fstat(AT_FDCWD) succeeds (it shouldn't) and returns information for the current directory

[adhemerval.zanella at linaro dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=27559

Adhemerval Zanella <adhemerval.zanella at linaro dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |adhemerval.zanella at linaro dot o
                   |                            |rg
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2021-03-10
           Assignee|unassigned at sourceware dot org   |adhemerval.zanella at linaro dot o
                   |                            |rg

--- Comment #1 from Adhemerval Zanella <adhemerval.zanella at linaro dot org> ---
Confirmed, now that we implement fstat with fstatat we should return EBADF for
negative input.

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[Bug libc/27559] fstat(AT_FDCWD) succeeds (it shouldn't) and returns information for the current directory

[adhemerval.zanella at linaro dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=27559

Adhemerval Zanella <adhemerval.zanella at linaro dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
   Target Milestone|---                         |2.34
         Resolution|---                         |FIXED

--- Comment #2 from Adhemerval Zanella <adhemerval.zanella at linaro dot org> ---
Fixed on 2.34 and I will backport to 2.33 as well.

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