man2/setfsgid.2

   1 .\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
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  25 .\" Created   1995-08-06 Thomas K. Dyas <tdyas@eden.rutgers.edu>
  26 .\" Modified  2000-07-01 aeb
  27 .\" Modified  2002-07-23 aeb
  28 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  29 .\"     Added notes on capability requirements
  30 .\"
  31 .TH SETFSGID 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  32 .SH NAME
  33 setfsgid \- set group identity used for filesystem checks
  34 .SH SYNOPSIS
  35 .B #include <sys/fsuid.h>
  36 .PP
  37 .BI "int setfsgid(uid_t " fsgid );
  38 .SH DESCRIPTION
  39 The system call
  40 .BR setfsgid ()
  41 changes the value of the caller's filesystem group ID\(emthe
  42 group ID that the Linux kernel uses to check for all accesses
  43 to the filesystem.
  44 Normally, the value of
  45 the filesystem group ID
  46 will shadow the value of the effective group ID.
  47 In fact, whenever the
  48 effective group ID is changed,
  49 the filesystem group ID
  50 will also be changed to the new value of the effective group ID.
  51 .PP
  52 Explicit calls to
  53 .BR setfsuid (2)
  54 and
  55 .BR setfsgid ()
  56 are usually used only by programs such as the Linux NFS server that
  57 need to change what user and group ID is used for file access without a
  58 corresponding change in the real and effective user and group IDs.
  59 A change in the normal user IDs for a program such as the NFS server
  60 is a security hole that can expose it to unwanted signals.
  61 (But see below.)
  62 .PP
  63 .BR setfsgid ()
  64 will succeed only if the caller is the superuser or if
  65 .I fsgid
  66 matches either the caller's real group ID, effective group ID,
  67 saved set-group-ID, or current the filesystem user ID.
  68 .SH RETURN VALUE
  69 On both success and failure,
  70 this call returns the previous filesystem group ID of the caller.
  71 .SH VERSIONS
  72 This system call is present in Linux since version 1.2.
  73 .\" This system call is present since Linux 1.1.44
  74 .\" and in libc since libc 4.7.6.
  75 .SH CONFORMING TO
  76 .BR setfsgid ()
  77 is Linux-specific and should not be used in programs intended
  78 to be portable.
  79 .SH NOTES
  80 Note that at the time this system call was introduced, a process
  81 could send a signal to a process with the same effective user ID.
  82 Today signal permission handling is slightly different.
  83 See
  84 .BR setfsuid (2)
  85 for a discussion of why the use of both
  86 .BR setfsuid (2)
  87 and
  88 .BR setfsgid ()
  89 is nowadays unneeded.
  90 .PP
  91 The original Linux
  92 .BR setfsgid ()
  93 system call supported only 16-bit group IDs.
  94 Subsequently, Linux 2.4 added
  95 .BR setfsgid32 ()
  96 supporting 32-bit IDs.
  97 The glibc
  98 .BR setfsgid ()
  99 wrapper function transparently deals with the variation across kernel versions.
 100 .SS C library/kernel differences
 101 In glibc 2.15 and earlier,
 102 when the wrapper for this system call determines that the argument can't be
 103 passed to the kernel without integer truncation (because the kernel
 104 is old and does not support 32-bit group IDs),
 105 it will return \-1 and set \fIerrno\fP to
 106 .B EINVAL
 107 without attempting
 108 the system call.
 109 .SH BUGS
 110 No error indications of any kind are returned to the caller,
 111 and the fact that both successful and unsuccessful calls return
 112 the same value makes it impossible to directly determine
 113 whether the call succeeded or failed.
 114 Instead, the caller must resort to looking at the return value
 115 from a further call such as
 116 .IR setfsgid(\-1)
 117 (which will always fail), in order to determine if a preceding call to
 118 .BR setfsgid ()
 119 changed the filesystem group ID.
 120 At the very
 121 least,
 122 .B EPERM
 123 should be returned when the call fails (because the caller lacks the
 124 .B CAP_SETGID
 125 capability).
 126 .SH SEE ALSO
 127 .BR kill (2),
 128 .BR setfsuid (2),
 129 .BR capabilities (7),
 130 .BR credentials (7)