Re: [PATCH] x86/bugs: Don't warn when overwriting retbleed_return_thunk with srso_return_thunk

On Thu, May 15, 2025 at 12:25:07AM +0200, Borislav Petkov wrote:

On Wed, May 14, 2025 at 03:08:35PM -0700, Suraj Jitindar Singh wrote:

...

• if (x86_return_thunk != __x86_return_thunk)

• /*

• * There can only be one return thunk enabled at a time, so issue a
  

• * warning when overwriting it. retbleed_return_thunk is a special case
  

• * which is safe to be overwritten with srso_return_thunk since it
  

• * provides a superset of the functionality and is handled correctly in
  

• * entry_untrain_ret().
  

• */
  

• if ((x86_return_thunk != __x86_return_thunk) &&

•    (thunk != srso_return_thunk ||
  

•     x86_return_thunk != retbleed_return_thunk))
  

Instead of making this an unreadable conditional, why don't we ...

...

pr_warn("x86/bugs: return thunk changed\n");

... turn this into a

pr_info("set return thunk to: %ps\n", ...)

and simply say which thunk was set?

This was discussed during the mitigation, and pr_warn() was chosen because it was not obvious that srso mitigation also mitigates retbleed. (On a retrospect, there should have been a comment about it).

The conclusion was to make the srso and retbleed relationship clear and then take care of the pr_warn().