https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70082
Bug ID: 70082
Summary: Attribute ifunc marked functions should not be allowed
to call other functions.
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: carlos at redhat dot com
Target Milestone: ---
In 2005 the GNU IFUNC support was documented and added to GCC via the ifunc
attribute. To be honest this was a mistake, the feature is not documented and
the implementation has so many caveats that it is incredibly difficult to use.
Recently gperftools added IFUNC usage in tcmalloc and they did a lot of things
which break the IFUNC implementation in general, but happy to work by luck on
x86_64.
I have documented some of this here:
https://sourceware.org/glibc/wiki/GNU_IFUNC
The IFUNC support is salvageable, but will require some reworking across the
various architectures to organize the relocations in a sensible way, and even
then it's going to be hard to allow everything. Particularly with -Wl,-z,now
which needs to complete all relocations early.
To that end, and until we fix IFUNC, the use of the ifunc attribute in a
function should immediately cause compilation to fail if that function calls
any other function. There is simply no way to guarantee you can make that
function call safely at early ld.so startup. You might allow calls to functions
in the same translation unit, but then you'd have to make sure those functions
don't also call other external functions. It's simply safer to limit the use to
something that works.
Is such a fix feasible?
