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patch-author-guide: add sibling call info

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kpatch-build doesn't support GCC's sibling call optimizations in patched
functions to avoid potentially dangerous generated code.  Explain the
error message that may result and link to the commit which provided
detailed explanation of why we disabled support for this in kpatch.

Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
This commit is contained in:
Joe Lawrence 2019-12-05 14:48:29 -05:00
parent 59fbf45d8e
commit cd305fd7ec
1 changed files with 23 additions and 0 deletions
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doc/patch-author-guide.md
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@ -687,3 +687,26 @@ if (static_key_enabled(&true_key))
if (static_key_enabled(&false_key))
if (likely(static_key_enabled(&key)))
```
Sibling calls
-------------
GCC may generate sibling calls that are incompatible with kpatch, resulting in
an error like: `ERROR("Found an unsupported sibling call at foo()+0x123. Add __attribute__((optimize("-fno-optimize-sibling-calls"))) to foo() definition."`
For example, if function A() calls function B() at the end of A() and both
return similar data-types, GCC may deem them "sibling calls" and apply a tail
call optimization in which A() restores the stack to is callee state before
setting up B()'s arguments and jumping to B().
This may be an issue for kpatches on PowerPC which modify only A() or B() and
the function call crosses a kernel module boundary: the sibling call
optimization has changed expected calling conventions and (un)patched code may
not be similarly modified.
Commit [8b952bd77130](https://github.com/dynup/kpatch/commit/8b952bd77130)
("create-diff-object/ppc64le: Don't allow sibling calls") contains an
excellent example and description of this problem with annotated disassembly.
Adding `__attribute__((optimize("-fno-optimize-sibling-calls")))` instructs
GCC to turn off the optimization for the given function.