On Thu, Apr 24, 2025 at 06:06:03PM +0000, Qing Zhao wrote:
>
>
> > On Apr 24, 2025, at 13:07, Kees Cook <k...@kernel.org> wrote:
> >
> > On Thu, Apr 24, 2025 at 04:36:14PM +0000, Qing Zhao wrote:
> >>
> >>> On Apr 24, 2025, at 11:59, Martin Uecker <uec...@tugraz.at> wrote:
> >>>
> >>> Am Donnerstag, dem 24.04.2025 um 15:15 +0000 schrieb Qing Zhao:
> >>>> Hi,
> >>>>
> >>>> Kees reported a segmentation failure when he used the patch to compiler
> >>>> kernel,
> >>>> and the reduced the testing case is something like the following:
> >>>>
> >>>> struct f {
> >>>> void *g __attribute__((__counted_by__(h)));
> >>>> long h;
> >>>> };
> >>>>
> >>>> extern struct f *my_alloc (int);
> >>>>
> >>>> int i(void) {
> >>>> struct f *iov = my_alloc (10);
> >>>> int *j = (int *)iov->g;
> >>>> return __builtin_dynamic_object_size(iov->g, 0);
> >>>> }
> >>>>
> >>>> Basically, the problem is relating to the pointee type of the pointer
> >>>> array being “void”,
> >>>> As a result, the element size of the array is not available in the IR.
> >>>> Therefore segmentation
> >>>> fault when calculating the size of the whole object.
> >>>>
> >>>> Although it’s easy to fix this segmentation failure, I am not quite sure
> >>>> what’s the best
> >>>> solution to this issue:
> >>>>
> >>>> 1. Reject such usage of “counted_by” in the very beginning by reporting
> >>>> warning to the
> >>>> User, and delete the counted_by attribute from the field.
> >>>>
> >>>> Or:
> >>>>
> >>>> 2. Accept such usage, but issue warnings when calculating the
> >>>> object_size in Middle-end.
> >>>>
> >>>> Personally, I prefer the above 1 since I think that when the pointee
> >>>> type is void, we don’t know
> >>>> The type of the element of the pointer array, there is no way to decide
> >>>> the size of the pointer array.
> >>>>
> >>>> So, the counted_by information is not useful for the
> >>>> __builtin_dynamic_object_size.
> >>>>
> >>>> But I am not sure whether the counted_by still can be used for bound
> >>>> sanitizer?
> >>>>
> >>>> Thanks for suggestions and help.
> >>>
> >>> GNU C allows pointer arithmetic and sizeof on void pointers and
> >>> that treats void as having size 1. So you could also allow counted_by
> >>> and assume as size 1 for void.
> >>>
> >>> https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html
> >>
> >> Okay, thanks for the info.
> >> So,
> >> 1. should we issue warnings when doing this?
> >
> > Please don't, Linux would very much like to track these allocation sizes
> > still. Performing pointer arithmetic and bounds checking (via __bdos) on
> > "void *" is wanted (and such a calculation was what tripped the
> > segfault).
>
> My previous question was: -:)
>
> When we accept the “void” pointee type and provide
> __builtin_dynamic_object_size for
> such pointers (treating it as 1 byte) shall we issue a warning to users to
> warn them that the void pointee type is
> Accepted and treated as size 1?
>
> Or just silently handle such case as normal?
I think it should be silently handled. Other such "void is 1 byte" cases
don't warn.
> >> 2. If the compilation option is explicitly asking for standard C,
> >> shall we issue warning and delete the counted_by attribute from the
> >> field?
> >
> > I think it needs to stay attached for __bdos. And from the looks of it,
> > even array access works with 1-byte values too:
> >
> > extern void *ptr;
> > void *foo(int num) {
> > return &ptr[num];
> > }
> >
> > The assembly output of this shows it's doing byte addition. Clang
> > doesn't warn about this, but GCC does:
> >
> > <source>:5:16: warning: dereferencing 'void *' pointer
> > 5 | return &ptr[num];
> > | ^
> >
> > So, I think even the bounds sanitizer should handle it, even if a
> > warning ultimately gets emitted.
>
> Okay. I will also handle the void in bounds sanitizer by treating element
> size as 1 byte.
Great, that should work well for Linux, and is, I think, the least
surprising result. :)
>
> My previous question was:
>
> Since this is only a GNU extension, I am wondering under the situation that
> No GNU extension
> Is allowed, shall we issue warnings and delete the counted_by attribute?
Oh, like, generally? What GCC option disables GNU extensions?
-Kees
--
Kees Cook
