execveat(2)

1execveat(2)                   System Calls Manual                  execveat(2)
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NAME

6       execveat - execute program relative to a directory file descriptor
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <linux/fcntl.h>      /* Definition of AT_* constants */
13       #include <unistd.h>
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15       int execveat(int dirfd, const char *pathname,
16                    char *const _Nullable argv[],
17                    char *const _Nullable envp[],
18                    int flags);
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DESCRIPTION

21       The execveat() system call executes the program referred to by the com‐
22       bination of dirfd and pathname.  It operates in exactly the same way as
23       execve(2), except for the differences described in this manual page.
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25       If  the  pathname given in pathname is relative, then it is interpreted
26       relative to the directory referred to  by  the  file  descriptor  dirfd
27       (rather  than  relative to the current working directory of the calling
28       process, as is done by execve(2) for a relative pathname).
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30       If pathname is relative and dirfd is the special value  AT_FDCWD,  then
31       pathname  is  interpreted  relative to the current working directory of
32       the calling process (like execve(2)).
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34       If pathname is absolute, then dirfd is ignored.
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36       If pathname is an empty string and the AT_EMPTY_PATH flag is specified,
37       then the file descriptor dirfd specifies the file to be executed (i.e.,
38       dirfd refers to an executable file, rather than a directory).
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40       The flags argument is a bit mask that can include zero or more  of  the
41       following flags:
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43       AT_EMPTY_PATH
44              If  pathname is an empty string, operate on the file referred to
45              by dirfd (which may have been obtained using the open(2)  O_PATH
46              flag).
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48       AT_SYMLINK_NOFOLLOW
49              If  the  file  identified  by dirfd and a non-NULL pathname is a
50              symbolic link, then the call fails with the error ELOOP.
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RETURN VALUE

53       On success, execveat() does not return.  On error, -1 is returned,  and
54       errno is set to indicate the error.
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ERRORS

57       The same errors that occur for execve(2) can also occur for execveat().
58       The following additional errors can occur for execveat():
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60       pathname
61              is relative but dirfd is neither AT_FDCWD nor a valid  file  de‐
62              scriptor.
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64       EINVAL Invalid flag specified in flags.
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66       ELOOP  flags  includes  AT_SYMLINK_NOFOLLOW  and the file identified by
67              dirfd and a non-NULL pathname is a symbolic link.
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69       ENOENT The program identified by dirfd and pathname requires the use of
70              an  interpreter  program  (such as a script starting with "#!"),
71              but the file descriptor dirfd  was  opened  with  the  O_CLOEXEC
72              flag,  with  the result that the program file is inaccessible to
73              the launched interpreter.  See BUGS.
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75       ENOTDIR
76              pathname is relative and dirfd is a file descriptor referring to
77              a file other than a directory.
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STANDARDS

80       Linux.
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HISTORY

83       Linux 3.19, glibc 2.34.
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NOTES

86       In  addition to the reasons explained in openat(2), the execveat() sys‐
87       tem call is also needed to allow fexecve(3) to be implemented  on  sys‐
88       tems that do not have the /proc filesystem mounted.
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90       When  asked to execute a script file, the argv[0] that is passed to the
91       script interpreter is a string of the form  /dev/fd/N  or  /dev/fd/N/P,
92       where N is the number of the file descriptor passed via the dirfd argu‐
93       ment.  A string of the first form  occurs  when  AT_EMPTY_PATH  is  em‐
94       ployed.   A  string of the second form occurs when the script is speci‐
95       fied via both dirfd and pathname; in this case, P is the value given in
96       pathname.
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98       For  the  same  reasons described in fexecve(3), the natural idiom when
99       using execveat() is to set the close-on-exec flag on dirfd.   (But  see
100       BUGS.)
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BUGS

103       The  ENOENT  error described above means that it is not possible to set
104       the close-on-exec flag on the file descriptor given to a  call  of  the
105       form:
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107           execveat(fd, "", argv, envp, AT_EMPTY_PATH);
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109       However,  the inability to set the close-on-exec flag means that a file
110       descriptor referring to the script leaks through to the script  itself.
111       As well as wasting a file descriptor, this leakage can lead to file-de‐
112       scriptor exhaustion in scenarios where scripts recursively  employ  ex‐
113       ecveat().
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