[Bug target/66491] New: x86_64 target cross-compiler generates stack protector code unsuitable for the Linux kernel if the compiler wasn't built against a C library

["dhowells at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66491

            Bug ID: 66491
           Summary: x86_64 target cross-compiler generates stack protector
                    code unsuitable for the Linux kernel if the compiler
                    wasn't built against a C library
           Product: gcc
           Version: 5.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dhowells at redhat dot com
  Target Milestone: ---

The Fedora gcc-5.1.1 cross-compiler targeting x86_64 doesn't generate
appropriate stack protection code as the compiler wasn't built against glibc.

For example, if the following command is run:

  echo "int foo(void) { char X[200]; return 3; }" |
  x86_64-linux-gnu-gcc -S -x c -c -O0 -mcmodel=kernel -fstack-protector - -o -

the code generated refers to __stack_chk_guard:
        ...
        movq    __stack_chk_guard(%rip), %rax
        movq    %rax, -8(%rbp)
        xorl    %eax, %eax
        movl    $3, %eax
        movq    -8(%rbp), %rdx
        xorq    __stack_chk_guard(%rip), %rdx
        je      .L3
        call    __stack_chk_fail
        ...

but it should instead use the %gs segment register to access the canary:
        ...
        movq    %gs:40, %rax
        movq    %rax, -8(%rbp)
        xorl    %eax, %eax
        movl    $3, %eax
        movq    -8(%rbp), %rdx
        xorq    %gs:40, %rdx
        je      .L3
        call    __stack_chk_fail
        ...

as is expected by the kernel.

This was originally logged in the Red Hat bugzilla as:

    https://bugzilla.redhat.com/show_bug.cgi?id=1228800

The reporter looked into how cross-gcc is built and the problem seems to be
this:
gcc is configured with "--without-headers". The configure script checks for
whether the target libc provides SSP (TARGET_LIBC_PROVIDES_SSP). In this case
it will be false.

The definition of TARGET_THREAD_SSP_OFFSET is conditional under "#ifdef
TARGET_LIBC_PROVIDES_SSP" (in gcc/config/i386/gnu-user64.h), so it will not be
defined.

config/i386/i386.md describes how to generate stack protector code. It will use
TLS canary only if "#ifdef TARGET_THREAD_SSP_OFFSET".

As per one of the reporter's suggestions, -mcmodel=kernel should perhaps switch
to the %gs method, overriding anything the gcc selects based on the C library.