RE: [PATCH] stack: fix pile GCC memcpy warnings

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> Sent: Tuesday, 20 October 2026 16.24
> 
> On Tue, 20 Oct 2026 08:49:44 +0000
> Morten Brørup <mb@smartsharesystems.com> wrote:
> 
> > On architectures where rte_memcpy() is defined as memcpy(),
> > GCC warns about source/destination overflow when copying more than
> the
> > source/destination size, even for code paths that are not taken in
> cases
> > where the sizes are smaller.
> >
> > The relevant compiler warnings were disabled for GCC at the two
> > occurrences, and comments describing the reason were added.
> >
> > NB:
> > It has not been investigated which GCC versions are affected.
> > GCC version 10 and up was chosen because that was what the previous
> > versions of x86 rte_memcpy() disabled sligthly similar warnings for.
> >
> > PS:
> > The alternative was to obfuscate the code, but that would make the
> code
> > less readable, and might have unintended effects on architectures and
> > compilers not suffering by this issue.
> >
> > Bugzilla ID: 2061
> > Fixes: ee0822a0ef41 ("stack: introduce pile")
> >
> > The pile was introduce with this DPDK release, so no need to
> backport.
> >
> > Reported-by: Ali Alnubani <alialnu@nvidia.com>
> > Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> > ---
> 
> I don't like using compiler pragmas for this. It also means that if
> compiler gets fixed or behaves differently you will never see it.
> 
> 
> AI analysis shows some other options.
> 
> Root cause is --buildtype=debug, i.e. -O0, not the GCC version. At -O0
> nothing removes the dead n_bulk > 0 path for rte_stack_push(s, &obj,
> 1), so the constant 256-byte memcpy is checked against obj (8 bytes)
> and warns. Reproduced with GCC 13.3 and 14.2 at -O0; silent at -Og and
> above. That means the GCC_VERSION >= 100000 gate is wrong at both ends:
> any GCC with the warning fires (overflow since 8, overread since 11),
> and on GCC 10 the -Wstringop-overread pragma itself is a -Wpragmas
> error under -Werror. If the pragma stays, drop the version gate (keep
> RTE_TOOLCHAIN_GCC) and fix the comment: GCC does not “know” the path is
> dead, that is the whole problem.

Continuing this line of thinking...
GCC defines an "__OPTIMIZE__" macro if any optimization above level 0 is turned on.
We could gate such pragmas with:
#if defined(RTE_TOOLCHAIN_GCC) && !defined(__OPTIMIZE__)

> 
> 
> This should have a big ass comment, and it should use same pragma set
> as existing one in lib/ring.

I generally prefer having the pragma coverage as narrow as possible, so they don't prevent catching unrelated bugs in the same file.

On the other hand,
I also like the suggestion in your other email - disabling these false-positive GCC warnings globally across DPDK for non-optimized builds.

And since the default build type is debugoptimized, it will not prevent GCC from emitting relevant warnings during normal development.

> 
> Other alternative is to use bulk object array as structure and do
> assignment.
> But that replaces ugly pragmas with ugly casts...

Yeah, let's not do that.

But thank you for exploring!

> 
> --- a/lib/stack/rte_stack_pile.h
> +++ b/lib/stack/rte_stack_pile.h
> @@
> +/* Bulk object array as a single object, so a copy is one assignment.
> */
> +struct rte_stack_pile_bulk_objs {
> +	void *objs[RTE_STACK_PILE_BULK_SIZE];
> +};
> +
> @@ __rte_stack_pile_bulk_pop_elems
>  		for (unsigned int i = 0; i < num; i++, tmp = tmp->next)
> -			rte_memcpy(&obj_table[i * RTE_STACK_PILE_BULK_SIZE],
> -					((struct rte_stack_pile_bulk_elem *)tmp)-
> >objs,
> -					sizeof(void *) *
> RTE_STACK_PILE_BULK_SIZE);
> +			*(struct rte_stack_pile_bulk_objs *)&obj_table[i *
> RTE_STACK_PILE_BULK_SIZE] =
> +				*(struct rte_stack_pile_bulk_objs *)((struct
> rte_stack_pile_bulk_elem *)tmp)->objs;
> @@ __rte_stack_pile_push
>  	for (i = 0; i < n_bulk; i++, tmp = tmp->next)
> -		rte_memcpy(((struct rte_stack_pile_bulk_elem *)tmp)->objs,
> -				&obj_table[(n_bulk - i - 1) *
> RTE_STACK_PILE_BULK_SIZE],
> -				sizeof(void *) * RTE_STACK_PILE_BULK_SIZE);
> +		*(struct rte_stack_pile_bulk_objs *)((struct
> rte_stack_pile_bulk_elem *)tmp)->objs =
> +			*(const struct rte_stack_pile_bulk_objs
> *)&obj_table[(n_bulk - i - 1) * RTE_STACK_PILE_BULK_SIZE];
>