Re: [PATCH] tree-object-size: Fix type-1 size for pointers to FAM-containing subobjects [PR126975]

On Fri, Aug 21, 2026 at 10:56:34AM +0200, Richard Biener wrote:
> On Fri, Aug 21, 2026 at 8:30 AM Gustavo A. R. Silva
> <[email protected]> wrote:
> >
> > For a pointer to a subobject whose record/union type ends in a flexible-array
> > member (directly, or through its trailing nested struct), addr_object_size()
> > walked up to the enclosing object (v = TREE_OPERAND (v, 0)) instead of
> > measuring the referenced subobject. __builtin_object_size() and
> > __builtin_dynamic_object_size() type 1 therefore returned the whole-object
> > size, collapsing type 1 onto type 0 and losing the distinction between
> > &p->inner and p. FORTIFY_SOURCE relies on the type-1 distinction, so this
> > weakens its bounds checks.
> >
> > Fix this by computing the size directly from the referenced record/union
> > instead of walking up, restoring the type-0/type-1 distinction that Clang
> > already implements.
> >
> > Bootstraped and regtested on x86_64-linux-gnu.
> >
> >         PR tree-optimization/126975
> >
> > gcc/ChangeLog:
> >
> >         * tree-object-size.cc (addr_object_size): For a reference to a
> >         record or union type, compute the object size from the referenced
> >         subobject instead of walking up to the enclosing object when the
> >         type recursively includes a flexible array member.
> >
> > gcc/testsuite/ChangeLog:
> >
> >         * gcc.dg/builtin-object-size-pr101832.c (main): Update the expected
> >         results of __builtin_object_size (..., 1) queries on subobjects
> >         whose type contains a flexible array member.
> > ---
> >  .../gcc.dg/builtin-object-size-pr101832.c      |  6 +++---
> >  gcc/tree-object-size.cc                        | 18 +++++-------------
> >  2 files changed, 8 insertions(+), 16 deletions(-)
> >
> > diff --git a/gcc/testsuite/gcc.dg/builtin-object-size-pr101832.c b/gcc/testsuite/gcc.dg/builtin-object-size-pr101832.c
> > index 60078e11634..d76286ae454 100644
> > --- a/gcc/testsuite/gcc.dg/builtin-object-size-pr101832.c
> > +++ b/gcc/testsuite/gcc.dg/builtin-object-size-pr101832.c
> > @@ -92,9 +92,9 @@ int main (int argc, char *argv[])
> >    outer = (void *)magic1;
> >    outest = (void *)magic2;
> >
> > -  expect (__builtin_object_size (&outer->a, 1), -1);
> > -  expect (__builtin_object_size (&outest->b, 1), -1);
> > -  expect (__builtin_object_size (&outest->b.a, 1), -1);
> > +  expect (__builtin_object_size (&outer->a, 1), sizeof(outer->a));
> 
> I think this warrants updating the documentation in extend.texi where
> I think the
> question is how for
> 
> struct A {
>   int n;
>   char data[];
> };
> 
> struct B {
>   int m;
>   struct A a;
> };
> 
> structr B *outer;
> 
> and object &outer->a, how, based on 'type', the enclosing object
> size is constrained by its enclosing object.  Also for outer->a.data
> what the enclosing object is and similar how the constraint from the
> enclosing objects are handled.
> 
> I'll note that __builtin_object_size behavior might be in conflict
> with constraints set by -fstrict-flex-arrays.  IIRC "nested" flex arrays
> are a GNU extension, flex arrays not at the end of an (enclusing)
> object as well (a particularly bad one).
> 
> You also need to update the toplevel comment of the testcase
> which explicitly says your change is wrong.

I actually think we don't want to change this, the handling of nested
flexible and flexible-like arrays has been completely intentional, it was
based on investigation of real-world code and finding some reasonable
middle-ground on what we consider already invalid and what we allow as an
extension, especially because it occurs a lot in real-world code.

What we certainly can change is the behavior when
-fstrict-flex-arrays unless it already behaves the expected way,
and perhaps also when the flex array has counted_by attribute.

Changing this unconditionally will just break a lot of software in the wild
when compiled with -O2 -D_FORTIFY_SOURCE=2 or -O2 -D_FORTIFY_SOURCE=3, which
is very common.

	Jakub