Use faccessat, not access, when checking file permissions.

This fixes a bug that has been present in Emacs since its creation.
It was reported by Chris Torek in 1983 even before GNU Emacs existed,
which must set some sort of record.  (Torek's bug report was against
a predecessor of GNU Emacs, but GNU Emacs happened to have the
same common flaw.)  See Torek's Usenet posting
"setuid/setgid programs & Emacs" Article-I.D.: sri-arpa.858
Posted: Fri Apr  8 14:18:56 1983.
* .bzrignore: Add lib/fcntl.h.
* configure.ac (euidaccess): Remove check; gnulib does this for us now.
(gl_FCNTL_O_FLAGS): Define a dummy version.
* lib/at-func.c, lib/euidaccess.c, lib/faccessat.c, lib/fcntl.in.h:
* lib/getgroups.c, lib/group-member.c, lib/root-uid.h:
* lib/xalloc-oversized.h, m4/euidaccess.m4, m4/faccessat.m4:
* m4/fcntl_h.m4, m4/getgroups.m4, m4/group-member.m4:
New files, from gnulib.
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.
* admin/merge-gnulib (GNULIB_MODULES): Add faccessat.
(GNULIB_TOOL_FLAGS): Avoid at-internal, fchdir, malloc-posix,
openat-die, openat-h, save-cwd.  Do not avoid fcntl-h.
Omit gnulib's m4/fcntl-o.m4.
* nt/inc/ms-w32.h (AT_FDCWD, AT_EACCESS): New symbols.
(access): Remove.
(faccessat): New macro.
* src/Makefile.in (LIB_EACCESS): New macro.
(LIBES): Use it.
* src/callproc.c (init_callproc):
* src/charset.c (init_charset):
* src/fileio.c (check_existing, check_executable, check_writable)
(Ffile_readable_p):
* src/lread.c (openp, load_path_check):
* src/process.c (allocate_pty):
* src/xrdb.c (file_p):
Use effective UID when checking permissions, not real UID.
* src/callproc.c (init_callproc):
* src/charset.c (init_charset):
* src/lread.c (load_path_check, init_lread):
Test whether directories are accessible, not merely whether they exist.
* src/conf_post.h (GNULIB_SUPPORT_ONLY_AT_FDCWD): New macro.
* src/fileio.c (check_existing, check_executable, check_writable)
(Ffile_readable_p):
Use symbolic names instead of integers for the flags, as they're
portable now.
(check_writable): New arg AMODE.  All uses changed.
Set errno on failure.
(Ffile_readable_p): Use faccessat, not stat + open + close.
(Ffile_writable_p): No need to call check_existing + check_writable.
Just call check_writable and then look at errno.  This saves a syscall.
dir should never be nil; replace an unnecessary runtime check
with an eassert.  When checking the parent directory of a nonexistent
file, check that the directory is searchable as well as writable, as
we can't create files in unsearchable directories.
(file_directory_p): New function, which uses 'stat' on most platforms
but faccessat with D_OK (for efficiency) if WINDOWSNT.
(Ffile_directory_p, Fset_file_times): Use it.
(file_accessible_directory_p): New function, which uses a single
syscall for efficiency.
(Ffile_accessible_directory_p): Use it.
* src/xrdb.c (file_p): Use file_directory_p.
* src/lisp.h (file_directory_p, file_accessible_directory_p): New decls.
* src/lread.c (openp): When opening a file, use fstat rather than
stat, as that avoids a permissions race.  When not opening a file,
use file_directory_p rather than stat.
(dir_warning): First arg is now a usage string, not a format.
Use errno.  All uses changed.
* src/nsterm.m (ns_term_init): Remove unnecessary call to file-readable
that merely introduced a race.
* src/process.c, src/sysdep.c, src/term.c: All uses of '#ifdef O_NONBLOCK'
changed to '#if O_NONBLOCK', to accommodate gnulib O_* style,
and similarly for the other O_* flags.
* src/w32.c (sys_faccessat): Rename from sys_access and switch to
faccessat's API.  All uses changed.
* src/xrdb.c: Do not include <sys/stat.h>; no longer needed.
(magic_db): Rename from magic_file_p.
(magic_db, search_magic_path): Return an XrmDatabase rather than a
char *, so that we don't have to test for file existence
separately from opening the file for reading.  This removes a race
fixes a permission-checking problem, and simplifies the code.
All uses changed.
(file_p): Remove; no longer needed.

Fixes: debbugs:12632

1 files changed, 2 insertions, 0 deletions

diff --git a/src/lisp.h b/src/lisp.h
index 72e38fa4653..67ae28a488f 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -3202,6 +3202,8 @@ extern Lisp_Object close_file_unwind (Lisp_Object);
 extern Lisp_Object restore_point_unwind (Lisp_Object);
 extern _Noreturn void report_file_error (const char *, Lisp_Object);
 extern void internal_delete_file (Lisp_Object);
+extern bool file_directory_p (const char *);
+extern bool file_accessible_directory_p (const char *);
 extern void syms_of_fileio (void);
 extern Lisp_Object make_temp_name (Lisp_Object, bool);
 extern Lisp_Object Qdelete_file;