34de4fe7d1
With CONFIG_RETHUNK enabled, the compiler replaces every RET with a tail
call to a return thunk ('JMP __x86_return_thunk'). Objtool annotates
all such return sites so they can be patched during boot by
apply_returns().
The implementation of __x86_return_thunk() is just a bare RET. It's
only meant to be used temporarily until apply_returns() patches all
return sites with either a JMP to another return thunk or an actual RET.
Removing the .text..__x86.return_thunk section would break objtool's
detection of return sites in retpolines. Since retpolines and return
thunks would land in the same section, the compiler no longer uses
relocations for the intra-section jumps between the retpolines and the
return thunk, causing objtool to overlook them.
As a result, none of the retpolines' return sites would get patched.
Each one stays at 'JMP __x86_return_thunk', effectively a bare RET.
Fix it by teaching objtool to detect when a non-relocated jump target is
a return thunk (or retpoline).
[ bp: Massage the commit message now that the offending commit
removing the .text..__x86.return_thunk section has been zapped.
Still keep the objtool change here as it makes objtool more robust
wrt handling such intra-TU jumps without relocations, should some
toolchain and/or config generate them in the future. ]
Reported-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20231012024737.eg5phclogp67ik6x@treble
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.