[Bug binutils/19421] [2.26 Regression] Missing weak symbols in kernel modules on powerpc64le-linux-gnu

[amodra at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=19421

Alan Modra <amodra at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at sourceware dot org   |amodra at gmail dot com

--- Comment #3 from Alan Modra <amodra at gmail dot com> ---
So far, I haven't been able to reproduce this problem using torvalds linux git
kernel and current binutils.

There are lots of claims in the debian bug report:

1)
fat: no symbol version for TOC.
fat: Unknown symbol TOC. (err -22)
ext4: no symbol version for TOC.
ext4: Unknown symbol TOC. (err -22)
and others like this.

This can happen if there is no entry for "TOC." in the __versions section of a
module, but I see for example
objdump -sj__versions fs/binfmt_misc.ko
...
 0e00 00000000 00000000 2e544f43 2e000000  .........TOC....
 0e10 00000000 00000000 00000000 00000000  ................
 0e20 00000000 00000000 00000000 00000000  ................
 0e30 00000000 00000000 00000000 00000000  ................
...
I picked on binfmt_misc.ko rather than the fat or ext4 modules since
binfmt_misc.ko is built for "make defconfig", but I see the same with ext4.ko
after flipping the appropriate .config lines.

2)
Changed section ordering for -z relro.  This shouldn't be a problem, but may be
one of the triggers for (4) below.

3)
Normally a ppc64el kernel has the following undefined symbols:
| 44809: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND mach_powermac
| 52870: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND mach_chrp
| 62021: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND mach_cell
| 62383: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __crc_TOC.

Also not a problem if these symbols disappear when undefined, I think.  The
mach_* symbols are used in the following macro
#define machine_is(name) \
        ({ \
                extern struct machdep_calls mach_##name \
                        __attribute__((weak));           \
                machine_id == &mach_##name; \
        })
A missing kernel symbol will be silently resolved to 0 if the reference is
STB_WEAK as far as I can see from my reading kernel/module.c.  This of course
is the same value if the symbol is found.  __crc_TOC. is similar (but I'm still
investigating this one).

4)
the mcount symbol error

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68759
If you're building with gcc-6, you will need a kernel patch.

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