Re: [PATCH] ubsan: Honor -fstrict-flex-arrays= in -fsanitize=bounds [PR108894]

[Qing Zhao <qing.zhao@oracle.com>]

> On Feb 28, 2023, at 4:59 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> 
> On Tue, Feb 28, 2023 at 07:19:40PM +0000, Qing Zhao wrote:
>> Understood.  
>> So, your patch fixed this bug, and then [0] arrays are instrumented by default with this patch.
>> 
>>> Well, it would complain about
>>> struct S { int a; int b[0]; int c; } s;
>>> ... &s.b[1] ...
>>> for C++, but not for C.
>> 
>> A little confused here: [0] arrays were instrumented by default for C++ if it’s not a trailing array, but not for C?
> 
> Given say
> struct S { int a; int b[0]; int c; } s;
> 
> int
> main ()
> {
>  int *volatile p = &s.b[0];
>  p = &s.b[1];
>  int volatile q = s.b[0];
> }
> both -fsanitize=bounds and -fsanitize=bounds-strict behaved the same way,
> in C nothing was reported, in C++ the p = &s.b[1]; statement.
> The reasons for s.b[0] not being reported in C++ was that for
> !ignore_off_by_one, bounds was ~(size_t)0, and so index > ~(size_t)0
> is always false.  While with the committed patch it is
> index >= (~(size_t)0)+1 and so always true.  And in C additionally, we
> punted early because TYPE_MAX_VALUE (domain) was NULL.

Thanks for the explanation.

With your patch, both C and C++ will report for the middle [0] arrays. That’s nice.

Qing
> 
> 	Jakub
>