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x86: Fix kthread unwind 

The rewrite of ret_from_form() misplaced an unwind hint which caused
all kthread stack unwinds to be marked unreliable, breaking
livepatching.

Restore the annotation and add a comment to explain the how and why of
things.

Fixes: 3aec4ecb3d ("x86: Rewrite ret_from_fork() in C")
Reported-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Petr Mladek <pmladek@suse.com>
Link: https://lkml.kernel.org/r/20230719201538.GA3553016@hirez.programming.kicks-ass.net
This commit is contained in:
Peter Zijlstra 2023-07-19 22:15:38 +02:00
parent fdf0eaf114
commit 2e7e5bbb1c
1 changed files with 15 additions and 1 deletions
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16
arch/x86/entry/entry_64.S
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@ -285,7 +285,15 @@ SYM_FUNC_END(__switch_to_asm)
*/
.pushsection .text, "ax"
SYM_CODE_START(ret_from_fork_asm)
UNWIND_HINT_REGS
/*
* This is the start of the kernel stack; even through there's a
* register set at the top, the regset isn't necessarily coherent
* (consider kthreads) and one cannot unwind further.
*
* This ensures stack unwinds of kernel threads terminate in a known
* good state.
*/
UNWIND_HINT_END_OF_STACK
ANNOTATE_NOENDBR // copy_thread
CALL_DEPTH_ACCOUNT
@ -295,6 +303,12 @@ SYM_CODE_START(ret_from_fork_asm)
movq %r12, %rcx /* fn_arg */
call ret_from_fork
/*
* Set the stack state to what is expected for the target function
* -- at this point the register set should be a valid user set
* and unwind should work normally.
*/
UNWIND_HINT_REGS
jmp swapgs_restore_regs_and_return_to_usermode
SYM_CODE_END(ret_from_fork_asm)
.popsection