Messages in this thread | | | From | James Y Knight <> | Date | Wed, 04 Apr 2018 19:26:06 +0000 | Subject | Re: [GIT PULL] x86/build changes for v4.17 |
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On Wed, Apr 4, 2018 at 12:59 PM Greg KH <gregkh@linuxfoundation.org> wrote: > Here is another horrible work around that was needed to get clang to > stop generating invalid code, beaec533fc27 ("llist: clang: introduce > member_address_is_nonnull()") That one caused a lot of odd failures by > users, I wonder what else is lurking in that same code pattern. It's a > hard one to debug...
I would note that this issue is an entirely different issue from the null-pointer-deref-is-undefined-behavior optimizations, even though it may seem superficially similar. For _this_ issue, the behavior at question is that the compiler assumes that objects are contiguous in memory, and thus that "&struct_pointer->member_at_nonzero_offset != NULL" is always true. I don't really see clang ever getting a flag to stop assuming that objects are contiguous.
Note that clang does actually emit an on-by-default warning for a situation analogous to this: warning: comparison of address of 'p->sub' not equal to a null pointer is always true [-Wtautological-pointer-compare] if (&p->sub != NULL) { ~~~^~~ ~~~~ ...but unfortunately that warning is suppressed when it occurs in a macro-expansion, so the llist_for_each_entry error was silent.
(OTOH, I _do_ expect clang to eventually gain support for a flag to treat NULL-pointer deref as a legal operation instead of UB. I'm not really sure it makes sense for the linux kernel, but it's a useful feature for the compiler to provide in any case, so why not...)
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