[PATCH] D134902: [clang] Implement -fstrict-flex-arrays=3

[serge via Phabricator via cfe-commits]

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.


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  https://reviews.llvm.org/D134902/new/

https://reviews.llvm.org/D134902