On 6/18/26 6:02 AM, Gustavo A. R. Silva wrote:

On 6/17/26 16:59, Gustavo A. R. Silva wrote:

quoted

On 6/17/26 16:01, Ilya Maximets wrote:

quoted

On 6/17/26 10:08 PM, Gustavo A. R. Silva wrote:

quoted

Hi,

On 6/16/26 04:03, Ilya Maximets wrote:

quoted

kmalloc_flex() in metadata_dst_alloc() sets __counted_by for the
structure to the options_len, which is then initialized to zero.
Later, we're initializing the structure by copying the tunnel info
together with the options, and this triggers a warning for a potential
memcpy overflow, since the compiler estimates that the options can't
fit into the structure, even though the memory for them is actually
allocated.

   memcpy: detected buffer overflow: 104 byte write of buffer size 96
   WARNING: CPU: X PID: Y at lib/string_helpers.c:1036 __fortify_report
    skb_tunnel_info_unclone+0x179/0x190
    geneve_xmit+0x7fe/0xe00

This warning has nothing to do with counted_by. See below for more
comments.

quoted

The issue is triggered when built with clang and source fortification.

Fix that by doing the copy in two stages: first - the main data with
the options_len, then the options.  This way the correct length should
be known at the time of the copy.

It would be better if the options_len never changed after allocation,
but the allocation code is a little separate from the initialization
and it would be awkward and potentially dangerous to return a struct
with options_len set to a non-zero value from the metadata_dst_alloc().

Another option would be to use ip_tunnel_info_opts_set(), but it is
doing too many unnecessary operations for the use case here.

Fixes: 69050f8d6d07 ("treewide: Replace kmalloc with kmalloc_obj for non-scalar types")
Reported-by: Johan Thomsen <redacted>
Closes: https://lore.kernel.org/netdev/CAKv6aAM8_EWgXScnKmKYm_4SwGDVBK++dzfP+Y6msUXbp99QUw@mail.gmail.com/ (local)
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
---

Johan, if you can test this one in your setup as well, that would
be great.  Thanks.

   include/net/dst_metadata.h | 7 +++++--
   1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/net/dst_metadata.h b/include/net/dst_metadata.h
index 1fc2fb03ce3f..f45d1e3163f0 100644
--- a/include/net/dst_metadata.h
+++ b/include/net/dst_metadata.h

@@ -164,8 +164,11 @@ static inline struct metadata_dst *tun_dst_unclone(struct sk_buff *skb)

       if (!new_md)
           return ERR_PTR(-ENOMEM);
-    memcpy(&new_md->u.tun_info, &md_dst->u.tun_info,
-           sizeof(struct ip_tunnel_info) + md_size);

What's going on here is that, internally, fortified memcpy() retrieves
the destination size via __builtin_dynamic_object_size() in mode 1.

That is:

__builtin_dynamic_object_size(&new_md->u.tun_info, 1)

For the above case, Clang returns sizeof(new_md->u.tun_info) == 96.

So the warning is reporting that 104 bytes don't fit in an object of
size 96 bytes, regardless of any counted_by annotation or allocation.

Hmm.  Does __builtin_dynamic_object_size(&new_md->u.tun_info, 1) return
104 when the options_len is 8?  If so, isn't that because it is counted
by that field?  Asking because the fortification doesn't complain if we
keep the full 104-byte copy as-is, but set the options_len beforehand,
as tested by Johan.

I see. If that is the case, then, internally, fortified memcpy() ends up
using mode 0 instead of mode 1. Something like this:

__builtin_dynamic_object_size(&new_md->u.tun_info, 0)

The above will effectively consider the allocation and counted_by because
it will interpret new_md->u.tun_info as an open-ended object due to the
flexible-array member (in struct ip_tunnel_info) whose size is determined
by counted_by.

Indeed. The execution stops here:

fortify_memcpy_chk():
588         /*
589          * Always stop accesses beyond the struct that contains the
590          * field, when the buffer's remaining size is known.
591          * (The SIZE_MAX test is to optimize away checks where the buffer
592          * lengths are unknown.)
593          */
594         if (p_size != SIZE_MAX && p_size < size)
595                 fortify_panic(func, FORTIFY_WRITE, p_size, size, true);

with p_size = __builtin_dynamic_object_size(&new_md->u.tun_info, 0)

The code never reaches the part where p_size_field (__bdos(&new_md->u.tun_info, 1))
is checked at runtime because there is no need for that.

So yep, this patch is okay as-is.

Ack.  Thanks for looking into this!

Best regards, Ilya Maximets.