Underaligned capabilities
This warning is triggered when packed structures contain pointers. As mentioned in Restrictions in capability locations in memory, pointers must always be aligned to the size of a CHERI capability (16 bytes for a 64-bit architecture). This warning can be triggered by code that attempts to align pointers to at least 8 bytes (e.g., for compatibility between 32- and 64-bit architectures). For example:
struct AtLeast8ByteAlignedBad {
void *data;
} __attribute__((packed, aligned(8)));
<source>:1:8: warning: alignment (8) of 'struct AtLeast8ByteAlignedBad' is
less than the required capability alignment (16) [-Wcheri-capability-misuse]
struct AtLeast8ByteAlignedBad {
^
<source>:1:8: note: If you are certain that this is correct you can silence
the warning by adding __attribute__((annotate("underaligned_capability")))
1 warning generated.
The simplest fix for this issue is to either increase alignment to be CHERI-compatible, or use a ternary expression to include alignof(void *):
#include <stdalign.h>
struct AtLeast8ByteAlignedGood {
void *data;
} __attribute__((packed,aligned(alignof(void *) > 8 ? alignof(void *) : 8)));
In the rare case that creating a potentially underaligned pointer is actually intended, the warning can be silence by adding a annotate("underaligned_capability") attribute:
struct UnderalignPointerIgnoreWarning {
void *data;
} __attribute__((packed, aligned(4), annotate("underaligned_capability")));