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From: "luto at kernel dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/98997] New: Linking mismatched -fcf-protection objects
 generates incorrect code
Date: Mon, 08 Feb 2021 06:15:36 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98997

            Bug ID: 98997
           Summary: Linking mismatched -fcf-protection objects generates
                    incorrect code
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: luto at kernel dot org
  Target Milestone: ---

$ gcc --version
gcc (GCC) 10.2.1 20201125 (Red Hat 10.2.1-9)
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cat cet1.c
#include <setjmp.h>

void test(jmp_buf buf)
{
        __builtin_longjmp(buf, 1);
}
$ cat cet2.c
#include <setjmp.h>

void test(jmp_buf buf);

int main()
{
        jmp_buf buf;
        if (__builtin_setjmp(buf) =3D=3D 0)
                test(buf);
        return 0;
}
$ gcc -c -O2 cet1.c -fcf-protection=3Dfull
$ gcc -c -O2 cet2.c
$ gcc -o cet -O2 cet1.o cet2.o
$ ./cet
Illegal instruction (core dumped)

Presumably the magic CET fixup in the builtin setjmp/longjmp requires that
matching code is generated for both.  I'm testing on a non-CET system.

I haven't even tried to figure out if anything sensible happens to the CET =
GNU
property, but I think the current GCC behavior is impolite at best.  I think
GCC should do one of a few things:

1. Generate working code.

2. Fail to link.

3. At least document this pitfall very clearly.

I really hope this doesn't affect shared objects.=