Ard Biesheuvel [off-list ref] writes:

On 11 April 2018 at 16:49, Michael Ellerman
[off-list ref] wrote:

quoted

On Tue, 2018-04-10 at 01:22:06 UTC, Michael Ellerman wrote:

quoted

If you build the kernel with CONFIG_RELOCATABLE=n, then install the
modules, rebuild the kernel with CONFIG_RELOCATABLE=y and leave the
old modules installed, we crash something like:

  Unable to handle kernel paging request for data at address 0xd000000018d66cef
  Faulting instruction address: 0xc0000000021ddd08
  Oops: Kernel access of bad area, sig: 11 [#1]
  Modules linked in: x_tables autofs4
  CPU: 2 PID: 1 Comm: systemd Not tainted 4.16.0-rc6-gcc_ubuntu_le-g99fec39 #1
  ...
  NIP check_version.isra.22+0x118/0x170
  Call Trace:
    __ksymtab_xt_unregister_table+0x58/0xfffffffffffffcb8 [x_tables] (unreliable)
    resolve_symbol+0xb4/0x150
    load_module+0x10e8/0x29a0
    SyS_finit_module+0x110/0x140
    system_call+0x58/0x6c

This happens because since commit 71810db27c1c ("modversions: treat
symbol CRCs as 32 bit quantities"), a relocatable kernel encodes and
handles symbol CRCs differently from a non-relocatable kernel.

Although it's possible we could try and detect this situation and
handle it, it's much more robust to simply make the state of
CONFIG_RELOCATABLE part of the module vermagic.

Fixes: 71810db27c1c ("modversions: treat symbol CRCs as 32 bit quantities")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Applied to powerpc fixes.

https://git.kernel.org/powerpc/c/73aca179d78eaa11604ba0783a6d8b

Thanks for the cc. I guess this only affects powerpc, given that it is
the only arch that switches between CRC immediate values and CRC
offsets depending on the configuration.

No worries.

Is there any reason we shouldn't always turn on CONFIG_MODULE_REL_CRCS?
It seems to work, but I wanted to test it more before switching to that,
hence the quick fix above.


arch/um looks like it might be switching based on config, but I don't
know enough to say:

  config LD_SCRIPT_STATIC
  	bool
  	default y
  	depends on STATIC_LINK
  
  config LD_SCRIPT_DYN
  	bool
  	default y
  	depends on !LD_SCRIPT_STATIC
          select MODULE_REL_CRCS if MODVERSIONS


cheers