diff mbox

libio: Eliminate _IO_stdin, _IO_stdout, _IO_stderr

Message ID 87va1hcoez.fsf@oldenburg2.str.redhat.com
State Committed
Headers

Commit Message

Florian Weimer Feb. 18, 2019, 11:43 a.m. UTC 
  * Gabriel F. T. Gomes:

>> diff --git a/libio/oldfileops.c b/libio/oldfileops.c
>> index 4d6c5e3fe7..10f2205e8a 100644
>> --- a/libio/oldfileops.c
>> +++ b/libio/oldfileops.c
>> @@ -109,10 +109,7 @@ _IO_old_file_init_internal (struct _IO_FILE_plus *fp)
>>  			     - (int) sizeof (struct _IO_FILE_complete));
>>    fp->file._fileno = -1;
>>  
>> -  if (__builtin_expect (&_IO_stdin_used != NULL, 1)
>> -      || (fp != (struct _IO_FILE_plus *) _IO_stdin
>> -	  && fp != (struct _IO_FILE_plus *) _IO_stdout
>> -	  && fp != (struct _IO_FILE_plus *) _IO_stderr))
>> +  if (&_IO_stdin_used != NULL && !_IO_legacy_file ((FILE *) fp))
>
> Why did the `||' become an `&&'?  Is this solving a bug?

No, this is was a mistake on my part. 8-( I did not want to change
behavior here.

The old code in _IO_old_file_init_internal looks like this:

  if (__builtin_expect (&_IO_stdin_used != NULL, 1)
      || (fp != (struct _IO_FILE_plus *) _IO_stdin
          && fp != (struct _IO_FILE_plus *) _IO_stdout
          && fp != (struct _IO_FILE_plus *) _IO_stderr))
    /* The object is dynamically allocated and large enough.  Initialize
       the _mode element as well.  */
    ((struct _IO_FILE_complete *) fp)->_mode = -1;

So in a new program (which defines _IO_stdin_used), we would assume the
_mode member is present.  In an old program (no _IO_stdin_used), we look
at _IO_stdin etc., which will have been initialized to point to
_IO_stdin_ etc.  If fp does not point to one of these objects, we write
to _mode.

So the condition matching the old code would indeed be:

  &_IO_stdin_used != NULL || !_IO_legacy_file ((FILE *) fp)

New patch below.

libio is somewhat inconsistent in the way it checks for legacy file
handles.  freopen uses the absence of _IO_stdin_used.  _IO_old_fopen
(the old implementation of fopen) does *not* look at _IO_stdin_used and
always allocates a large struct, which includes the _mode member:

$ gdb libio/oldiofopen.os
…
(gdb) print sizeof(struct _IO_FILE_complete_plus)
$1 = 152

Compare this to:

$ gdb libio/oldstdfiles.os
…
(gdb) print sizeof(_IO_stdin_)
$1 = 80

Presumably, this makes the assignment to _mode safe in all cases where
the object is not one of the pre-allocated _IO_stdin_ (etc.) objects.
It is not safe if a shared object brings its own statically allocated
FILE object (no matter how it is named) and the shared object is loaded
into a new application (which defines _IO_stdin_used).  This object will
not have a _mode member, yet, _IO_old_file_init_internal tries to assign
to it.

We know that _IO_stdin_used is unreliable (see my reply to Dmitry).  The
question is whether we can find a representative set of old binaries to
make changes in this area.

Thanks,
Florian

libio: Eliminate _IO_stdin, _IO_stdout, _IO_stderr

These variables are only used to determine if a stdio stream is
a pre-allocated stream, but it is possible to do so by comparing
a FILE * to all pre-allocated stream objects.  As a result, it is
not necessary to keep those pointers in separate variables.

Behavior with symbol interposition is unchanged because _IO_stdin_,
_IO_stdout_, _IO_stderr_ are exported, and refer to objects outside of
libc if symbol interposition or copy relocations are involved.  (The
removed variables _IO_stdin, _IO_stdout, _IO_stderr were not exported,
of course.)

2019-02-18  Florian Weimer  <fweimer@redhat.com>

	* libio/libio.h (_IO_stdin, _IO_stdout, _IO_stderr): Remove
	declaration.
	* libio/stdio.c (AL, AL2, _IO_stdin, _IO_stdout, _IO_stderr):
	Remove definitions.
	* libio/stdfiles.c: Update comment.
	* libio/oldstdfiles.c (_IO_check_libio): Update comment.  Do not
	set _IO_stdin, _IO_stdout, _IO_stderr.
	* libio/libioP.h (_IO_fake_stdiobuf): Remove unused declaration.
	[SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)] (_IO_legacy_file): New
	inline function.
	(_IO_deallocate_file): New inline function.
	* libio/iolibio.h (_IO_vprintf): Remove definition.
	* libio/iofclose.c (_IO_new_fclose): Use _IO_deallocate_file.
	* libio/oldiofclose.c (_IO_old_fclose): Likewise.
	* libio/iofwide.c (_IO_fwide): Use __glibc_unlikely and
	_IO_legacy_file.
	* libio/oldfileops.c (_IO_old_file_init_internal): Remove
	__builtin_expect.  Use _IO_legacy_file.

Comments

Dmitry V. Levin Feb. 18, 2019, 12:54 p.m. UTC | #1 
On Mon, Feb 18, 2019 at 12:43:32PM +0100, Florian Weimer wrote:
[...]
> These variables are only used to determine if a stdio stream is
> a pre-allocated stream, but it is possible to do so by comparing
> a FILE * to all pre-allocated stream objects.  As a result, it is
> not necessary to keep those pointers in separate variables.
> 
> Behavior with symbol interposition is unchanged because _IO_stdin_,
> _IO_stdout_, _IO_stderr_ are exported, and refer to objects outside of
> libc if symbol interposition or copy relocations are involved.  (The
> removed variables _IO_stdin, _IO_stdout, _IO_stderr were not exported,
> of course.)
> 
> 2019-02-18  Florian Weimer  <fweimer@redhat.com>
> 
> 	* libio/libio.h (_IO_stdin, _IO_stdout, _IO_stderr): Remove
> 	declaration.
> 	* libio/stdio.c (AL, AL2, _IO_stdin, _IO_stdout, _IO_stderr):
> 	Remove definitions.
> 	* libio/stdfiles.c: Update comment.
> 	* libio/oldstdfiles.c (_IO_check_libio): Update comment.  Do not
> 	set _IO_stdin, _IO_stdout, _IO_stderr.
> 	* libio/libioP.h (_IO_fake_stdiobuf): Remove unused declaration.
> 	[SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)] (_IO_legacy_file): New
> 	inline function.
> 	(_IO_deallocate_file): New inline function.
> 	* libio/iolibio.h (_IO_vprintf): Remove definition.
> 	* libio/iofclose.c (_IO_new_fclose): Use _IO_deallocate_file.
> 	* libio/oldiofclose.c (_IO_old_fclose): Likewise.
> 	* libio/iofwide.c (_IO_fwide): Use __glibc_unlikely and
> 	_IO_legacy_file.
> 	* libio/oldfileops.c (_IO_old_file_init_internal): Remove
> 	__builtin_expect.  Use _IO_legacy_file.

Looks good, thanks.
Gabriel F. T. Gomes Feb. 18, 2019, 3:06 p.m. UTC | #2 
On Mon, Feb 18 2019, Florian Weimer wrote:
> 
> So in a new program (which defines _IO_stdin_used), we would assume the
> _mode member is present.  In an old program (no _IO_stdin_used), we look
> at _IO_stdin etc., which will have been initialized to point to
> _IO_stdin_ etc.  If fp does not point to one of these objects, we write
> to _mode.
>
> ...
>
> libio is somewhat inconsistent in the way it checks for legacy file
> handles.  freopen uses the absence of _IO_stdin_used.  _IO_old_fopen
> (the old implementation of fopen) does *not* look at _IO_stdin_used and
> always allocates a large struct, which includes the _mode member:
> 
> $ gdb libio/oldiofopen.os
> …
> (gdb) print sizeof(struct _IO_FILE_complete_plus)
> $1 = 152
> 
> Compare this to:
> 
> $ gdb libio/oldstdfiles.os
> …
> (gdb) print sizeof(_IO_stdin_)
> $1 = 80
> 
> Presumably, this makes the assignment to _mode safe in all cases where
> the object is not one of the pre-allocated _IO_stdin_ (etc.) objects.
> It is not safe if a shared object brings its own statically allocated
> FILE object (no matter how it is named) and the shared object is loaded
> into a new application (which defines _IO_stdin_used).  This object will
> not have a _mode member, yet, _IO_old_file_init_internal tries to assign
> to it.

Thanks for the detailed explanations.

> libio: Eliminate _IO_stdin, _IO_stdout, _IO_stderr
> 
> These variables are only used to determine if a stdio stream is
> a pre-allocated stream, but it is possible to do so by comparing
> a FILE * to all pre-allocated stream objects.  As a result, it is
> not necessary to keep those pointers in separate variables.
> 
> Behavior with symbol interposition is unchanged because _IO_stdin_,
> _IO_stdout_, _IO_stderr_ are exported, and refer to objects outside of
> libc if symbol interposition or copy relocations are involved.  (The
> removed variables _IO_stdin, _IO_stdout, _IO_stderr were not exported,
> of course.)

OK.

> -  if (__builtin_expect (&_IO_stdin_used != NULL, 1)
> -      || (fp != (struct _IO_FILE_plus *) _IO_stdin
> -	  && fp != (struct _IO_FILE_plus *) _IO_stdout
> -	  && fp != (struct _IO_FILE_plus *) _IO_stderr))
> +  if (&_IO_stdin_used != NULL || !_IO_legacy_file ((FILE *) fp))

OK.

This version looks good to me. Thanks!
diff mbox

Patch

diff --git a/libio/iofclose.c b/libio/iofclose.c
index 9b39a6cc4e..8a80dd0b78 100644
--- a/libio/iofclose.c
+++ b/libio/iofclose.c
@@ -71,12 +71,7 @@  _IO_new_fclose (FILE *fp)
       if (_IO_have_backup (fp))
 	_IO_free_backup_area (fp);
     }
-  if (fp != _IO_stdin && fp != _IO_stdout && fp != _IO_stderr)
-    {
-      fp->_flags = 0;
-      free(fp);
-    }
-
+  _IO_deallocate_file (fp);
   return status;
 }
 
diff --git a/libio/iofwide.c b/libio/iofwide.c
index 6676ad5e53..247cfde3d0 100644
--- a/libio/iofwide.c
+++ b/libio/iofwide.c
@@ -87,8 +87,7 @@  _IO_fwide (FILE *fp, int mode)
   mode = mode < 0 ? -1 : (mode == 0 ? 0 : 1);
 
 #if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
-  if (__builtin_expect (&_IO_stdin_used == NULL, 0)
-      && (fp == _IO_stdin || fp == _IO_stdout || fp == _IO_stderr))
+  if (__glibc_unlikely (&_IO_stdin_used == NULL) && _IO_legacy_file (fp))
     /* This is for a stream in the glibc 2.0 format.  */
     return -1;
 #endif
diff --git a/libio/iolibio.h b/libio/iolibio.h
index 2642d71e4f..9561833655 100644
--- a/libio/iolibio.h
+++ b/libio/iolibio.h
@@ -58,8 +58,6 @@  extern int _IO_vsscanf (const char *, const char *, __gnuc_va_list) __THROW;
    == _IO_pos_BAD ? EOF : 0)
 #define _IO_rewind(FILE) \
   (void) _IO_seekoff_unlocked (FILE, 0, 0, _IOS_INPUT|_IOS_OUTPUT)
-#define _IO_vprintf(FORMAT, ARGS) \
-  _IO_vfprintf (_IO_stdout, FORMAT, ARGS)
 #define _IO_freopen(FILENAME, MODE, FP) \
   (_IO_file_close_it (FP), \
    _IO_file_fopen (FP, FILENAME, MODE, 1))
diff --git a/libio/libio.h b/libio/libio.h
index 9c08c7eba1..c38095ff77 100644
--- a/libio/libio.h
+++ b/libio/libio.h
@@ -185,9 +185,6 @@  struct _IO_FILE_plus;
 extern struct _IO_FILE_plus _IO_2_1_stdin_;
 extern struct _IO_FILE_plus _IO_2_1_stdout_;
 extern struct _IO_FILE_plus _IO_2_1_stderr_;
-extern FILE *_IO_stdin attribute_hidden;
-extern FILE *_IO_stdout attribute_hidden;
-extern FILE *_IO_stderr attribute_hidden;
 
 struct _IO_cookie_file;
 
diff --git a/libio/libioP.h b/libio/libioP.h
index 8c75f15167..1c434ec3a1 100644
--- a/libio/libioP.h
+++ b/libio/libioP.h
@@ -817,7 +817,35 @@  extern int _IO_vscanf (const char *, va_list) __THROW;
 # endif
 #endif
 
-extern struct _IO_fake_stdiobuf _IO_stdin_buf, _IO_stdout_buf, _IO_stderr_buf;
+#if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
+/* See oldstdfiles.c.  These are the old stream variables.  */
+extern struct _IO_FILE_plus _IO_stdin_;
+extern struct _IO_FILE_plus _IO_stdout_;
+extern struct _IO_FILE_plus _IO_stderr_;
+
+static inline bool
+_IO_legacy_file (FILE *fp)
+{
+  return fp == (FILE *) &_IO_stdin_ || fp == (FILE *) &_IO_stdout_
+    || fp == (FILE *) &_IO_stderr_;
+}
+#endif
+
+/* Deallocate a stream if it is heap-allocated.  Preallocated
+   stdin/stdout/stderr streams are not deallocated. */
+static inline void
+_IO_deallocate_file (FILE *fp)
+{
+  /* The current stream variables.  */
+  if (fp == (FILE *) &_IO_2_1_stdin_ || fp == (FILE *) &_IO_2_1_stdout_
+      || fp == (FILE *) &_IO_2_1_stderr_)
+    return;
+#if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
+  if (_IO_legacy_file (fp))
+    return;
+#endif
+  free (fp);
+}
 
 #ifdef IO_DEBUG
 # define CHECK_FILE(FILE, RET) do {			\
diff --git a/libio/oldfileops.c b/libio/oldfileops.c
index 4d6c5e3fe7..5c319beacd 100644
--- a/libio/oldfileops.c
+++ b/libio/oldfileops.c
@@ -109,10 +109,7 @@  _IO_old_file_init_internal (struct _IO_FILE_plus *fp)
 			     - (int) sizeof (struct _IO_FILE_complete));
   fp->file._fileno = -1;
 
-  if (__builtin_expect (&_IO_stdin_used != NULL, 1)
-      || (fp != (struct _IO_FILE_plus *) _IO_stdin
-	  && fp != (struct _IO_FILE_plus *) _IO_stdout
-	  && fp != (struct _IO_FILE_plus *) _IO_stderr))
+  if (&_IO_stdin_used != NULL || !_IO_legacy_file ((FILE *) fp))
     /* The object is dynamically allocated and large enough.  Initialize
        the _mode element as well.  */
     ((struct _IO_FILE_complete *) fp)->_mode = -1;
diff --git a/libio/oldiofclose.c b/libio/oldiofclose.c
index e4cbf88566..be5044c3bd 100644
--- a/libio/oldiofclose.c
+++ b/libio/oldiofclose.c
@@ -58,12 +58,7 @@  _IO_old_fclose (FILE *fp)
   _IO_FINISH (fp);
   if (_IO_have_backup (fp))
     _IO_free_backup_area (fp);
-  if (fp != _IO_stdin && fp != _IO_stdout && fp != _IO_stderr)
-    {
-      fp->_flags = 0;
-      free(fp);
-    }
-
+  _IO_deallocate_file (fp);
   return status;
 }
 
diff --git a/libio/oldstdfiles.c b/libio/oldstdfiles.c
index 524e260b7e..2b861cd754 100644
--- a/libio/oldstdfiles.c
+++ b/libio/oldstdfiles.c
@@ -27,11 +27,8 @@ 
 #include <shlib-compat.h>
 #if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
 
-/* This file provides definitions of _IO_stdin, _IO_stdout, and _IO_stderr
-   for C code.  Compare stdstreams.cc.
-   (The difference is that here the vtable field is set to 0,
-   so the objects defined are not valid C++ objects.  On the other
-   hand, we don't need a C++ compiler to build this file.) */
+/* This file provides legacy definitions of _IO_stdin_, _IO_stdout_,
+   and _IO_stderr_.  See stdfiles.c for the current definitions.  */
 
 #define _IO_USE_OLD_IO_FILE
 #include "libioP.h"
@@ -78,13 +75,12 @@  _IO_check_libio (void)
   if (&_IO_stdin_used == NULL)
     {
       /* We are using the old one. */
-      _IO_stdin = stdin = (FILE *) &_IO_stdin_;
-      _IO_stdout = stdout = (FILE *) &_IO_stdout_;
-      _IO_stderr = stderr = (FILE *) &_IO_stderr_;
+      stdin = (FILE *) &_IO_stdin_;
+      stdout = (FILE *) &_IO_stdout_;
+      stderr = (FILE *) &_IO_stderr_;
       _IO_list_all = &_IO_stderr_;
-      _IO_stdin->_vtable_offset = _IO_stdout->_vtable_offset =
-	_IO_stderr->_vtable_offset = stdin->_vtable_offset =
-	stdout->_vtable_offset = stderr->_vtable_offset =
+      stdin->_vtable_offset = stdout->_vtable_offset
+	= stderr->_vtable_offset =
 	((int) sizeof (struct _IO_FILE)
 	 - (int) sizeof (struct _IO_FILE_complete));
     }
diff --git a/libio/stdfiles.c b/libio/stdfiles.c
index 605e006474..9c779b47eb 100644
--- a/libio/stdfiles.c
+++ b/libio/stdfiles.c
@@ -25,11 +25,10 @@ 
    in files containing the exception.  */
 
 
-/* This file provides definitions of _IO_stdin, _IO_stdout, and _IO_stderr
-   for C code.  Compare stdstreams.cc.
-   (The difference is that here the vtable field is set to 0,
-   so the objects defined are not valid C++ objects.  On the other
-   hand, we don't need a C++ compiler to build this file.) */
+/* This file provides definitions of _IO_2_1_stdin_, _IO_2_1_stdout_,
+   and _IO_2_1_stderr_, the default values of stdin, stdout, stderr.
+   See oldstdfiles.c for glibc 2.0 legacy definitions without wide
+   character support.  */
 
 #include "libioP.h"
 
diff --git a/libio/stdio.c b/libio/stdio.c
index 1294d2842e..522de44a27 100644
--- a/libio/stdio.c
+++ b/libio/stdio.c
@@ -33,14 +33,3 @@ 
 FILE *stdin = (FILE *) &_IO_2_1_stdin_;
 FILE *stdout = (FILE *) &_IO_2_1_stdout_;
 FILE *stderr = (FILE *) &_IO_2_1_stderr_;
-
-#undef _IO_stdin
-#undef _IO_stdout
-#undef _IO_stderr
-#define AL(name) AL2 (name, _IO_##name)
-#define AL2(name, al) \
-  extern __typeof (name) al __attribute__ ((alias (#name),                    \
-                                            visibility ("hidden")))
-AL(stdin);
-AL(stdout);
-AL(stderr);