[Bug libc/20680] New: ifunc resolver cannot access the thread pointer with static linking

["nszabolcs at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=20680

            Bug ID: 20680
           Summary: ifunc resolver cannot access the thread pointer with
                    static linking
           Product: glibc
           Version: 2.24
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: nszabolcs at gmail dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

the thread pointer is not yet initialized when ifunc resolvers are
run with static linking.

this means -fstack-protect-all instrumented resolver crashes with
static linking on targets where the canary is in tcb.
(can affect any tcb offset abi the compiler may use, like the
hwcap/platform on powerpc.)

and any tls access crashes the resolver (accessing errno or using
tls with global dynamic model can crash with dynamic linking too
because they are extern calls, see bug 20673).

it is generally not documented what c code may work before
thread-pointer setup.

compile with -static
$ cat main.c
static int foo1(void) { return 1; }
static int foo2(void) { return 2; }

__thread int x; // or try -fstack-protect-all

static int (*foo_resolver())()
{
        return x ? foo1 : foo2;
}

int foo(void) __attribute__((ifunc("foo_resolver")));

int main()
{
        foo();
}

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