Differential D3043 Diff 6985 lib/libc/secure/__builtin_object_size.3

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lib/libc/secure/__builtin_object_size.3

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				.\" $NetBSD: __builtin_object_size.3,v 1.10 2012/07/19 06:44:12 wiz Exp $
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				.Dd July 18, 2012
				.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 gcc 1
				built-in function that returns the size of the
				.Fa ptr
				object if known at compile time and the object does not have any side
				effects.
				kibUnsubmitted Not Done Inline Actions What does it mean that the object does not have any side effects ? Is this a typo ? Or does it indeed means that the object must be PDO (doubt it) ? kib: What does it mean that the object does not have any side effects ? Is this a typo ? Or does it…
				.Sh RETURN VALUES
				If the size of the object is not known or it has side effects the
				.Fn __builtin_object_size
				pfgUnsubmitted Not Done Inline Actions the "side effects" concept is surely vague: this manpage comes from NetBSD and it looks like it came from the GCC documentation: https://gcc.gnu.org/onlinedocs/gcc/Object-Size-Checking.html It doesn't really make sense for us to document compiler features. It is preferable to create a new fortify_source(7) man page explaining the effect of -D_FORTIFY_SOURCE. pfg: the "side effects" concept is surely vague: this manpage comes from NetBSD and it looks like it…
				jillesUnsubmitted Not Done Inline Actions I think "side effects" are things like function calls and writes to variables in the `ptr` expression. However, I think this manual page documenting a pure compiler feature should not be added. jilles: I think "side effects" are things like function calls and writes to variables in the `ptr`…
				pfgUnsubmitted Not Done Inline Actions I have been rethinking this and documenting `__builtin_object_size` only makes sense if we document the difference between GCC and clang. Those are important in {our\|\|Android} implementation and I have only seen those documented here https://copperhead.co/2015/07/27/hardening-bionic Of course this is to be done in the context of a wider fortify_source documentation. pfg: I have been rethinking this and documenting `__builtin_object_size` only makes sense if we…
				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 that they can be pointed to by
				.Fa ptr
				when
				.Fa type
				.Dv \*[Am] 2 == 0 ,
				and the minimum size when
				.Fa type
				.Dv \*[Am] 2 != 0 .
				.Sh SEE ALSO
				.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 currently code fortification (size-checked replacement
				functions) is disabled when optimization is off.