Messages in this thread | | | Date | Thu, 26 Mar 2020 14:00:01 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v4 01/13] objtool: Remove CFI save/restore special case |
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On Thu, Mar 26, 2020 at 12:30:50PM +0100, Peter Zijlstra wrote: > Subject: objtool: Remove CFI save/restore special case > From: Peter Zijlstra <peterz@infradead.org> > Date: Wed Mar 25 12:58:16 CET 2020 > > There is a special case in the UNWIND_HINT_RESTORE code. When, upon > looking for the UNWIND_HINT_SAVE instruction to restore from, it finds > the instruction hasn't been visited yet, it normally issues a WARN, > except when this HINT_SAVE instruction is the first instruction of > this branch. > > The reason for this special case comes apparent when we remove it; > code like: > > if (cond) { > UNWIND_HINT_SAVE > // do stuff > UNWIND_HINT_RESTORE > } > // more stuff > > will now trigger the warning. This is because UNWIND_HINT_RESTORE is > just a label, and there is nothing keeping it inside the (extended) > basic block covered by @cond. It will attach itself to the first > instruction of 'more stuff' and we'll hit it outside of the @cond, > confusing things. > > I don't much like this special case, it confuses things and will come > apart horribly if/when the annotation needs to support nesting. > Instead extend the affected code to at least form an extended basic > block.
> @@ -727,6 +727,13 @@ static inline void sync_core(void) > #else > unsigned int tmp; > > + /* > + * The trailing NOP is required to make this an extended basic block, > + * such that we can argue about it locally. Specifically this is > + * important for the UNWIND_HINTs, without this the UNWIND_HINT_RESTORE > + * can fall outside our extended basic block and objtool gets > + * (rightfully) confused. > + */ > asm volatile ( > UNWIND_HINT_SAVE > "mov %%ss, %0\n\t" > @@ -739,7 +746,7 @@ static inline void sync_core(void) > "pushq $1f\n\t" > "iretq\n\t" > UNWIND_HINT_RESTORE > - "1:" > + "1: nop\n\t" > : "=&r" (tmp), ASM_CALL_CONSTRAINT : : "cc", "memory");
Note that the special case very much relies on the HINT_SAVE being the first instruction of the (extended) basic block, which is only true in this one usage anyway.
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