On 11/30/2023 5:15 PM, Casey Schaufler wrote:

On 11/30/2023 12:30 AM, Petr Tesarik wrote:

quoted

Hi all,

On 11/30/2023 1:41 AM, Casey Schaufler wrote:

quoted

...
It would be nice if the solution directly addresses the problem.
EVM needs to be after the LSMs that use xattrs, not after all LSMs.
I suggested LSM_ORDER_REALLY_LAST in part to identify the notion as
unattractive.

Excuse me to chime in, but do we really need the ordering in code?

tl;dr - Yes.

quoted

  FWIW
the linker guarantees that objects appear in the order they are seen
during the link (unless --sort-section overrides that default, but this
option is not used in the kernel). Since *.a archive files are used in
kbuild, I have also verified that their use does not break the
assumption; they are always created from scratch.

In short, to enforce an ordering, you can simply list the corresponding
object files in that order in the Makefile. Of course, add a big fat
warning comment, so people understand the order is not arbitrary.

Not everyone builds custom kernels.

Sorry, I didn't understand your comment. Everyone builds the kernel, 
also Linux distros. What Petr was suggesting was that it does not matter 
how you build the kernel, the linker will place the LSMs in the order 
they appear in the Makefile. And for this particular case, we have:

obj-$(CONFIG_IMA)                       += ima/
obj-$(CONFIG_EVM)                       += evm/

In the past, I also verified that swapping these two resulted in the 
swapped order of LSMs. Petr confirmed that it would always happen.

Thanks

Roberto