0298739b79
The ftrace_regs_caller() trampoline does something 'funny' when there is a direct-caller present. In that case it stuffs the 'direct-caller' address on the return stack and then exits the function. This then results in 'returning' to the direct-caller with the exact registers we came in with -- an indirect tail-call without using a register. This however (rightfully) confuses objtool because the function shares a few instruction in order to have a single exit path, but the stack layout is different for them, depending through which path we came there. This is currently cludged by forcing the stack state to the non-direct case, but this generates actively wrong (ORC) unwind information for the direct case, leading to potential broken unwinds. Fix this issue by fully separating the exit paths. This results in having to poke a second RET into the trampoline copy, see ftrace_regs_caller_ret. This brings us to a second objtool problem, in order for it to perceive the 'jmp ftrace_epilogue' as a function exit, it needs to be recognised as a tail call. In order to make that happen, ftrace_epilogue needs to be the start of an STT_FUNC, so re-arrange code to make this so. Finally, a third issue is that objtool requires functions to exit with the same stack layout they started with, which is obviously violated in the direct case, employ the new HINT_RET_OFFSET to tell objtool this is an expected exception. Together, this results in generating correct ORC unwind information for the ftrace_regs_caller() function and it's trampoline copies. Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Miroslav Benes <mbenes@suse.cz> Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com> Acked-by: Josh Poimboeuf <jpoimboe@redhat.com> Link: https://lkml.kernel.org/r/20200416115118.749606694@infradead.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
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.