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 appropriately.
31

ERRORS

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

45       fexecve() is implemented since glibc 2.3.2.
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ATTRIBUTES

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

CONFORMING TO

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

NOTES

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

BUGS

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

SEE ALSO

103       execve(2), execveat(2)
104

COLOPHON

106       This page is part of release 5.10 of the Linux  man-pages  project.   A
107       description  of  the project, information about reporting bugs, and the
108       latest    version    of    this    page,    can     be     found     at
109       https://www.kernel.org/doc/man-pages/.
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113Linux                             2019-10-10                        FEXECVE(3)
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