On Thu, Jan 11, 2024 at 10:15:40AM +0300, Dan Carpenter wrote: > On Wed, Jan 10, 2024 at 04:03:28PM -0800, Kees Cook wrote: > > > > Oops, yes, thanks for fixing my confusion. Right, this is a direct write > > across members into the flex array, not a composite destination. Yay > > all the corner cases. :P > > Is there a document somewhere which explains what will trigger a runtime > warning? For example, if you write across members but it's not into a > flex array does that cause an issue? Or if you read across members?
There isn't a good place to find this. There are some code comments near the memcpy macros, but that's not really sufficient. At present FORTIFY is only being pedantic about _writes_, as that's generally a higher priority problem. The implemented restriction is that the destination buffer must be a single addressable struct member. That destination can be a composite member (i.e. an entire substruct), but going beyond a single member in a single memcpy() is no longer allowed because the compiler cannot reason about whether such a copy is "intentional". > For example, this line reads from bulletin->vlan and > bulletin->vlan_padding. This causes a compiler warning in Clang and > GCC depending on the options but does it also trigger a runtime warning? > https://elixir.bootlin.com/linux/latest/source/drivers/net/ethernet/broadcom/bnx2x/bnx2x_sriov.c#L2655 Right, the -Wstringop-overread and related compiler flags are doing the source buffer size checking. Note that for the compile-time warnings, GCC has the capacity to be much more strict than the FORTIFY checks because it can perform value _range_ tracking, where as FORTIFY compile-time checks are limited to having the copy size being a constant expression. (i.e. GCC may produce compile time warnings for cases that FORTIFY will only warn about at runtime if the range is violated.) > (I wrote a patch for this a few months back but didn't send it because > of the merge window. I forgot about it until now that we're in a merge > window again... :P) memcpy(&ivi->vlan, &bulletin->vlan, VLAN_HLEN); #define VLAN_HLEN 4 ivi->vlan is u32 bulletin has: u16 vlan; u8 vlan_padding[6]; yeah, ew. Should it even be reading padding? i.e. should this be: ivi->vlan = bulletin->vlan << 16; ? Or should bulletin be: union { struct { u16 vlan; u8 vlan_padding[6]; }; struct { u32 vlan_header; u8 vlan_header_padding[4]; }; }; with: ivi->vlan = bulletin->vlan_header; ? I've been finding that almost all memcpy()s and memset()s into non-array types are better just rewritten as a direct assignment. :P -- Kees Cook
