serge-sans-paille added inline comments.
================
Comment at: clang/test/CodeGen/object-size-flex-array.c:45
+ // CHECK-STRICT-2: ret i32 -1
+ // CHECK-STRICT-3: ret i32 0
return OBJECT_SIZE_BUILTIN(f->c, 1);
----------------
kees wrote:
> serge-sans-paille wrote:
> > This one worries me a bit, as an array of size 0 is invalid C, unless you
> > consider the extension of it being a FAM. Shouldn't we emit a warning or
> > something here? @kees what meaning would you give to that construct under
> > `-fstrict-flex-arrays=3` ?
> ```
> type identifier[0]
> ```
> when not a fake FAM is the same as:
>
> ```
> struct { } identifier
> ```
>
> It's addressable with no size.
>
> FWIW, this is how GCC is treating it, and opted for no warning. The warning
> gains nothing and is likely an irritant: if someone is requesting =3, they
> want this behavior. If they didn't, they'd just use `-Wzero-length-array`. In
> particular, the Linux kernel is in the position of needing to have
> zero-length arrays (legacy userspace API) alongside real FAMs, even if we
> don't reference the zero-length members. So we cannot use
> '-Wzero-length-array', but we want to make sure no zero-length arrays will
> ever be used as fake FAMs, as a code quality/style enforcement. This is
> especially true of FORTIFY_SOURCE, where `__bos()` needs to report 0 instead
> of -1 for such a destination buffer size, so that we immediately trip
> compile-time (or at worst, run-time) warnings, to keep any kernel internals
> from using the deprecated members as a fake FAM.
>
> Take this case:
>
> ```
> struct broken {
> int foo;
> int fake_fam[0];
> struct something oops;
> };
> ```
>
> There have been bugs where the above struct was created because "oops" got
> added after "fake_fam" by someone not realizing. Under FORTIFY_SOURCE, doing:
>
> ```
> memcpy(p->fake_fam, src, len);
> ```
>
> raises no warning when `__bos(p->fake_fam, 1)` returns -1 and will happily
> stomp on "oops". If `__bos()` returns 0, we can compile-time (or run-time)
> block the memcpy. (And this holds for -fsanitize=bounds as well: if it is
> considered to be unknown size, it won't trip on access, but if it's 0-sized,
> it'll trip.)
>
> So, we can't keep zero-length arrays out of the kernel, but we want to be
> able to enforce that if they DO show up, they will trip warnings quickly.
>
Thanks for clarifying the situation, esp. the kernel background. This looks
like a quite specific scenario, but it's fine with me.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D134902/new/
https://reviews.llvm.org/D134902
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