The expected semantics and valid usage of __builtin_return_address is
not clear since it exposes implementation internals that are normally
not meaningful to portable c code.
This documentation change tries to clarify the semantics in case the
return address is stored in a mangled form. This affects AArch64 when
pointer authentication is used for the return address signing (i.e.
-mbranch-protection=pac-ret).
gcc/ChangeLog:
2020-06-04 Szabolcs Nagy <szabolcs.n...@arm.com>
* doc/extend.texi: Update the text for __builtin_return_address.
---
gcc/doc/extend.texi | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index cced19d2018..0fd32a22599 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -11151,18 +11151,30 @@ The @var{level} argument must be a constant integer.
On some machines it may be impossible to determine the return address of
any function other than the current one; in such cases, or when the top
-of the stack has been reached, this function returns @code{0} or a
-random value. In addition, @code{__builtin_frame_address} may be used
+of the stack has been reached, this function returns an unspecified
+value. In addition, @code{__builtin_frame_address} may be used
to determine if the top of the stack has been reached.
Additional post-processing of the returned value may be needed, see
@code{__builtin_extract_return_addr}.
+The stored representation of the return address in memory may be different
+from the address returned by @code{__builtin_return_address}. For example
+on AArch64 the stored address may be mangled with return address signing.
+
Calling this function with a nonzero argument can have unpredictable
effects, including crashing the calling program. As a result, calls
that are considered unsafe are diagnosed when the @option{-Wframe-address}
option is in effect. Such calls should only be made in debugging
situations.
+
+On targets where code addresses are representable as @code{void *},
+@smallexample
+void *addr = __builtin_extract_return_addr (__builtin_return_address (0))
+@end smallexample
+gives the code address where the current function would return. For example
+such address may be used with @code{dladdr} or other interfaces that work
+with code addresses.
@end deftypefn
@deftypefn {Built-in Function} {void *} __builtin_extract_return_addr (void
*@var{addr})
--
2.17.1
