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

[Kees Cook via Phabricator via cfe-commits]

kees 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);
----------------
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.



Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D134902/new/

https://reviews.llvm.org/D134902