man2/setfsgid.2

   1 .\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
   2 .\" and Copyright (C) 2019, Michael Kerrisk <mtk.manpages@gmail.com>
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  26 .\" Created   1995-08-06 Thomas K. Dyas <tdyas@eden.rutgers.edu>
  27 .\" Modified  2000-07-01 aeb
  28 .\" Modified  2002-07-23 aeb
  29 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  30 .\"     Added notes on capability requirements
  31 .\"
  32 .TH SETFSGID 2 2019-05-09 "Linux" "Linux Programmer's Manual"
  33 .SH NAME
  34 setfsgid \- set group identity used for filesystem checks
  35 .SH SYNOPSIS
  36 .B #include <sys/fsuid.h>
  37 .PP
  38 .BI "int setfsgid(uid_t " fsgid );
  39 .SH DESCRIPTION
  40 On Linux, a process has both a filesystem group ID and an effective group ID.
  41 The (Linux-specific) filesystem group ID is used
  42 for permissions checking when accessing filesystem objects,
  43 while the effective group ID is used for some other kinds
  44 of permissions checks (see
  45 .BR credentials (7)).
  46 .PP
  47 Normally, the value of the process's filesystem group ID
  48 is the same as the value of its effective group ID.
  49 This is so, because whenever a process's effective group ID is changed,
  50 the kernel also changes the filesystem group ID to be the same as
  51 the new value of the effective group ID.
  52 A process can cause the value of its filesystem group ID to diverge
  53 from its effective group ID by using
  54 .BR setfsgid ()
  55 to change its filesystem group ID to the value given in
  56 .IR fsgid .
  57 .PP
  58 .BR setfsgid ()
  59 will succeed only if the caller is the superuser or if
  60 .I fsgid
  61 matches either the caller's real group ID, effective group ID,
  62 saved set-group-ID, or current the filesystem user ID.
  63 .SH RETURN VALUE
  64 On both success and failure,
  65 this call returns the previous filesystem group ID of the caller.
  66 .SH VERSIONS
  67 This system call is present in Linux since version 1.2.
  68 .\" This system call is present since Linux 1.1.44
  69 .\" and in libc since libc 4.7.6.
  70 .SH CONFORMING TO
  71 .BR setfsgid ()
  72 is Linux-specific and should not be used in programs intended
  73 to be portable.
  74 .SH NOTES
  75 The filesystem group ID concept and the
  76 .BR setfsgid ()
  77 system call were invented for historical reasons that are
  78 no longer applicable on modern Linux kernels.
  79 See
  80 .BR setfsuid (2)
  81 for a discussion of why the use of both
  82 .BR setfsuid (2)
  83 and
  84 .BR setfsgid ()
  85 is nowadays unneeded.
  86 .PP
  87 The original Linux
  88 .BR setfsgid ()
  89 system call supported only 16-bit group IDs.
  90 Subsequently, Linux 2.4 added
  91 .BR setfsgid32 ()
  92 supporting 32-bit IDs.
  93 The glibc
  94 .BR setfsgid ()
  95 wrapper function transparently deals with the variation across kernel versions.
  96 .SS C library/kernel differences
  97 In glibc 2.15 and earlier,
  98 when the wrapper for this system call determines that the argument can't be
  99 passed to the kernel without integer truncation (because the kernel
 100 is old and does not support 32-bit group IDs),
 101 it will return \-1 and set \fIerrno\fP to
 102 .B EINVAL
 103 without attempting
 104 the system call.
 105 .SH BUGS
 106 No error indications of any kind are returned to the caller,
 107 and the fact that both successful and unsuccessful calls return
 108 the same value makes it impossible to directly determine
 109 whether the call succeeded or failed.
 110 Instead, the caller must resort to looking at the return value
 111 from a further call such as
 112 .IR setfsgid(\-1)
 113 (which will always fail), in order to determine if a preceding call to
 114 .BR setfsgid ()
 115 changed the filesystem group ID.
 116 At the very
 117 least,
 118 .B EPERM
 119 should be returned when the call fails (because the caller lacks the
 120 .B CAP_SETGID
 121 capability).
 122 .SH SEE ALSO
 123 .BR kill (2),
 124 .BR setfsuid (2),
 125 .BR capabilities (7),
 126 .BR credentials (7)