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    SubjectRe: [PATCH 6/8] i915,uaccess: Fix redundant CLAC
    On Thu, Feb 28, 2019 at 08:01:11PM +0100, Peter Zijlstra wrote:
    > On Thu, Feb 28, 2019 at 10:29:25AM -0800, Linus Torvalds wrote:
    > > On Thu, Feb 28, 2019 at 10:02 AM Peter Zijlstra <peterz@infradead.org> wrote:
    > > >
    > > > Weird, that jump is from C, not from a .fixup table. objtool _should_
    > > > see that and complain if there is a AC=1 path that reaches RET.
    > >
    > > No, unsafe_put_user() actually does the "asm goto" thing, so the jump
    > > is literally hidden as an exception entry. And apparently objtool
    > > doesn't follow exceptions (which *normally* doesn't matter for code
    > > liveness analysis since they normally jump back to right after the
    > > excepting instruction, but maybe it misses some exception handling
    > > code because of it?).
    > >
    > > You may have looked at unsafe_get_user(), which does indeed make the
    > > branch as C code, because gcc currently does not allow outputs from
    > > "asm goto" statements (which "get" obviously needs).
    >
    > Indeed I did. But it looks like objtool actually does parse .fixup. What
    > appears to go wrong is the 'visited' marker for backward jumps.
    >
    > If we've been there with AC=0 first, and then backjump with AC=1, things
    > go missing.
    >
    > I've also now confused myself on how it branches from alternatives. It
    > looks like it now considers paths that take the STAC alternative, and
    > exit through the NOP alternative (which should be CLAC) and then hit
    > RET with AC=1.
    >
    > I'll get this sorted, eventually..

    Ha!

    Original file:

    CC drivers/gpu/drm/i915/i915_gem_execbuffer.o
    drivers/gpu/drm/i915/i915_gem_execbuffer.o: warning: objtool: .altinstr_replacement+0x3c: redundant UACCESS disable
    drivers/gpu/drm/i915/i915_gem_execbuffer.o: warning: objtool: .altinstr_replacement+0x66: redundant UACCESS disable

    With the dodgy patch:

    CC drivers/gpu/drm/i915/i915_gem_execbuffer.o
    drivers/gpu/drm/i915/i915_gem_execbuffer.o: warning: objtool: eb_relocate_slow()+0x1f9: call to kvfree() with UACCESS enabled
    drivers/gpu/drm/i915/i915_gem_execbuffer.o: warning: objtool: i915_gem_execbuffer2_ioctl()+0x315: call to kvfree() with UACCESS enabled


    Let me do an allmodconfig build to see how much pain is caused by that
    redundant CLAC warning.

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