HardenedBSD/lib/libssp/__builtin_object_size.3
Kyle Evans be04fec426 Import _FORTIFY_SOURCE implementation from NetBSD
This is a mostly-unmodified copy of the various *_chk implementations
and headers from NetBSD, without yet modifying system headers to start
actually including them.  A future commit will also apply the needed
bits to fix ssp/unistd.h.

Reviewed by:	imp, pauamma_gundo.com (both previous versions), kib
Sponsored by:	Stormshield
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D32306
2024-05-13 00:23:49 -05:00

111 lines
3.3 KiB
Groff

.\" $NetBSD: __builtin_object_size.3,v 1.11 2017/07/03 21:32:49 wiz Exp $
.\"
.\" SPDX-License-Identifier: BSD-2-Clause
.\"
.\" Copyright (c) 2007 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Christos Zoulas.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\"
.Dd April 27, 2024
.Dt __BUILTIN_OBJECT_SIZE 3
.Os
.Sh NAME
.Nm __builtin_object_size
.Nd return the size of the given object
.Sh SYNOPSIS
.Ft size_t
.Fn __builtin_object_size "void *ptr" "int type"
.Sh DESCRIPTION
The
.Fn __builtin_object_size
function is a
.Xr clang 1
and
.Xr gcc 1
built-in function that returns the size of the object referenced by
.Fa ptr
if known at compile time.
If the
.Fa ptr
expression has any side effects, then they will not be evaluated.
.Sh RETURN VALUES
If the size of the object is not known or the
.Fa ptr
expression has side effects, the
.Fn __builtin_object_size
function returns:
.Bl -tag -width (size_t)\-1 -offset indent
.It Dv (size_t)\-1
for
.Fa type
.Dv 0
and
.Dv 1 .
.It Dv (size_t)0
for
.Fa type
.Dv 2
and
.Dv 3 .
.El
.Pp
If the size of the object is known, then the
.Fn __builtin_object_size
function returns the maximum size of all the objects that the compiler
knows can be pointed to by
.Fa ptr
when
.Fa type
.Dv & 2 == 0 ,
and the minimum size when
.Fa type
.Dv & 2 != 0 .
.Sh SEE ALSO
.Xr clang 1 ,
.Xr gcc 1 ,
.Xr __builtin_return_address 3 ,
.Xr attribute 3 ,
.Xr ssp 3
.Sh HISTORY
The
.Fn __builtin_object_size
appeared in
.Tn GCC 4.1 .
.Sh CAVEATS
This is a non-standard, compiler-specific extension.
.Pp
Note that currently the object size calculation pass is only done at -O1
or above, meaning that this function always returns \-1 when the optimizer
is off.
.Pp
There are some discussions about always doing the object size pass, but
the issue is that without the optimization pass data sizes are not going
to be correct.
.Pp
For that reason code fortification (size-checked replacement
functions) is currently disabled when optimization is off.