Re: [ndctl PATCH v3 1/8] ndctl/build: Suppress -Waddress-of-packed-member

On Mon, Aug 5, 2019 at 10:06 AM Jeff Moyer <jmoyer(a)redhat.com&gt; wrote: > > Dan Williams <dan.j.williams(a)intel.com&gt; writes: > > > gcc 9.1.1 emits a slew of warnings for many of the command field > > accesses. I.e. warnings of the form: > > > > libndctl.c:2586:21: warning: taking address of packed member of ‘struct nd_cmd_get_config_data_hdr’ may result in an unaligned pointer value [-Waddress-of-packed-member] > > 2586 | cmd->iter.offset = &cmd->get_data->in_offset; > > | ^~~~~~~~~~~~~~~~~~~~~~~~~ > > > > Suppress these as fixing the warning would defeat the abstraction of being able > > to have common code that operates on commands with common fields at different > > offsets in the payload. > > As I understand it, taking a pointer to this potentially unaligned > member can result in bus errors (or worse, accessing wrong data) on > architectures that don't support unaligned accesses. I'd be a whole lot > happier with this changelog if it mentioned that you had considered what > the warning actually meant, and decided it didn't matter for the > architectures you want to support. True, it was a fleeting thought, but not something I considered fleshing out... I'll send a revision. > > x86 is, of course, safe. I believe aarch64 is, as well. I didn't look > into others. > Keep in mind that this code is for interfacing with the ACPI DSM path. If an unaligned-incapable architecture defined an NVDIMM command set it is highly unlikely it would be ACPI based, or pick these problematic command formats. I can add these notes to the changelog, but the unfortunate definition of these payloads that require __packed is something I hope other architectures avoid.