Messages in this thread | | | Subject | Re: drivers/net/ethernet/sfc/selftest.c:48:16: warning: field ip within 'struct efx_loopback_payload::(anonymous at drivers/net/ethernet/sfc/selftest.c:46:2)' is less aligned than 'struct iphdr' and is usually due to 'struct efx_loopback_payload::(anonym | From | Edward Cree <> | Date | Wed, 22 Nov 2023 16:15:49 +0000 |
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On 21/11/2023 21:25, kernel test robot wrote: > Hi Edward, > > FYI, the error/warning still remains.
As I've argued previously, this is a false positive / compiler bug, and there is no way to resolve it without making the code strictly worse.
This: >>> drivers/net/ethernet/sfc/selftest.c:48:16: warning: field ip within 'struct efx_loopback_payload::(anonymous at drivers/net/ethernet/sfc/selftest.c:46:2)' is less aligned than 'struct iphdr' and is usually due to 'struct efx_loopback_payload::(anonymous at drivers/net/ethernet/sfc/selftest.c:46:2)' being packed, which can lead to unaligned accesses [-Wunaligned-access] is complaining about alignment within an anonymous struct, which only ever appears embedded within a larger struct in a way which maintains the correct alignment.
#ifdef RANT
Indeed, the only way we even *could* create an unaligned access out of this code would be via a declaration like typeof(*(((struct efx_loopback_payload *)0)->packet)) bad; because *the struct is anonymous*. And if that happened, the bad declaration would be the place to warn, both because it's incredibly ugly and because it's the place that's actually wrong. The struct definition itself is entirely *fine*. The compiler should be able to detect that, and if it's not smart enough to do so then it shouldn't be trying to warn in the first place. Quoth Linus[1]:
"And if the compiler isn't good enough to do it, then the compiler shouldn't be warning about something that it hasn't got a clue about."
The anonymous struct has to be there so that we can placate the memcpy hardening, and it has to contain a struct iphdr at a 4n+2 offset because that's what shape the on-the-wire packet *is*. To avoid the warning we would need to lose __packed and memcpy all of the members in and out of the buffer individually to explicitly-calculated offsets, which is worse code.
#endif
Either fix the compiler to not warn, or fix your automation to ignore this instance of the warning.
-ed
[1]: https://yarchive.net/comp/linux/gcc.html#13
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