1FEXECVE(3)                 Linux Programmer's Manual                FEXECVE(3)
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NAME

6       fexecve - execute program specified via file descriptor
7

SYNOPSIS

9       #include <unistd.h>
10
11       int fexecve(int fd, char *const argv[], char *const envp[]);
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13   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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15       fexecve():
16           Since glibc 2.10:
17               _POSIX_C_SOURCE >= 200809L
18           Before glibc 2.10:
19               _GNU_SOURCE
20

DESCRIPTION

22       fexecve() performs the same task as execve(2), with the difference that
23       the file to be executed is specified via a file descriptor, fd,  rather
24       than  via  a pathname.  The file descriptor fd must be opened read-only
25       (O_RDONLY) or with the O_PATH flag and the caller must have  permission
26       to execute the file that it refers to.
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RETURN VALUE

29       A  successful  call to fexecve() never returns.  On error, the function
30       does return, with a result value of -1, and errno is  set  to  indicate
31       the error.
32

ERRORS

34       Errors are as for execve(2), with the following additions:
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36       EINVAL fd  is  not a valid file descriptor, or argv is NULL, or envp is
37              NULL.
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39       ENOENT The close-on-exec flag is set on fd, and fd refers to a  script.
40              See BUGS.
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42       ENOSYS The kernel does not provide the execveat(2) system call, and the
43              /proc filesystem could not be accessed.
44

VERSIONS

46       fexecve() is implemented since glibc 2.3.2.
47

ATTRIBUTES

49       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
50       tributes(7).
51
52       ┌────────────────────────────────────────────┬───────────────┬─────────┐
53Interface                                   Attribute     Value   
54       ├────────────────────────────────────────────┼───────────────┼─────────┤
55fexecve()                                   │ Thread safety │ MT-Safe │
56       └────────────────────────────────────────────┴───────────────┴─────────┘
57

CONFORMING TO

59       POSIX.1-2008.   This  function is not specified in POSIX.1-2001, and is
60       not  widely  available  on  other  systems.    It   is   specified   in
61       POSIX.1-2008.
62

NOTES

64       On Linux with glibc versions 2.26 and earlier, fexecve() is implemented
65       using the proc(5) filesystem, so /proc needs to be mounted  and  avail‐
66       able at the time of the call.  Since glibc 2.27, if the underlying ker‐
67       nel supports the execveat(2) system call, then fexecve() is implemented
68       using that system call, with the benefit that /proc does not need to be
69       mounted.
70
71       The idea behind fexecve() is to allow the caller to  verify  (checksum)
72       the  contents of an executable before executing it.  Simply opening the
73       file, checksumming the contents, and then doing an execve(2) would  not
74       suffice,  since,  between  the  two steps, the filename, or a directory
75       prefix of the pathname, could have been  exchanged  (by,  for  example,
76       modifying  the target of a symbolic link).  fexecve() does not mitigate
77       the problem that the contents of a file could be  changed  between  the
78       checksumming  and  the  call to fexecve(); for that, the solution is to
79       ensure that the permissions on the file prevent it from being  modified
80       by malicious users.
81
82       The natural idiom when using fexecve() is to set the close-on-exec flag
83       on fd, so that the file descriptor does not leak through to the program
84       that is executed.  This approach is natural for two reasons.  First, it
85       prevents file descriptors being consumed unnecessarily.  (The  executed
86       program  normally  has  no need of a file descriptor that refers to the
87       program itself.)  Second, if fexecve() is used  recursively,  employing
88       the  close-on-exec  flag  prevents  the file descriptor exhaustion that
89       would result from the fact that each step in the recursion would  cause
90       one  more  file  descriptor  to be passed to the new program.  (But see
91       BUGS.)
92

BUGS

94       If fd refers to a script (i.e., it is  an  executable  text  file  that
95       names a script interpreter with a first line that begins with the char‐
96       acters #!)  and the close-on-exec flag has been set for fd,  then  fex‐
97       ecve()  fails with the error ENOENT.  This error occurs because, by the
98       time the script interpreter is executed, fd has already been closed be‐
99       cause of the close-on-exec flag.  Thus, the close-on-exec flag can't be
100       set on fd if it refers to a script, leading to the  problems  described
101       in NOTES.
102

SEE ALSO

104       execve(2), execveat(2)
105

COLOPHON

107       This  page  is  part of release 5.12 of the Linux man-pages project.  A
108       description of the project, information about reporting bugs,  and  the
109       latest     version     of     this    page,    can    be    found    at
110       https://www.kernel.org/doc/man-pages/.
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114Linux                             2021-03-22                        FEXECVE(3)
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