Messages in this thread | | | Date | Wed, 12 Nov 2014 13:29:29 -0600 | From | Jeff Epler <> | Subject | Re: [PATCH] arch: tile: kernel: kgdb.c: Use memcpy() instead of pointer copy one by one |
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On Wed, Nov 12, 2014 at 11:43:08PM +0800, Chen Gang wrote: > > (I assume the > > compiler could do things like replace an intended load from memory with > > a constant load or even no load at all) > > > > Excuse me, my English is not quite well, I can not understand what you > said above. (If necessary, please help provide more details for it).
I am concerned that writing regs[TREG_TP] is "undefined behavior" according to the C standard.
This expression is equivalent to *(regs + TREG_TP). The expression (regs + TREG_TP) does not result in a pointer to any element of regs[], so dereferencing it is undefined behavior. (Source: C99 draft standard WG14/N1256, annex J.2, "[The behavior is undefined if t]he operand of the unary * operator has an invalid value")
That is why the compiler showed the original diagnostic, but the same logic that made the loop's behavior undefined also makes the expression regs[TREG_TP] undefined whereever it appears.
None of this is a specific problem with your proposed patch. Rather, it is a suggestion that the whole structure's design needs to be revisited in light of compilers beginning to notice that regs[TREG_TP] is undefined behavior and change their generated code as a result.
Unfortunately it looks like this header is also a part of the userspace API, so it can't simply be changed just in case all in-kernel uses are changed.
Jeff
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