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engine=8.12.0-2505160000 definitions=main-2507040125 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwNzA0MDEyNSBTYWx0ZWRfX2i9+4tCtMRZK 1Hh2205KxJfh9DbNWCw6ShzEjdsOBgPeylvjjfHXqBSxrEcfz8RXjK09t/wVpM/0Ha/27+pFTyg ls/LtB5y/phdiBNdhb4t5IP8IYfnMEbDfBH5wi3QK1K+Ty3oPj5KdUdqjL/OZvn5leFVzG4PFYU ZPSNEiWcfet2ACzI+hRifcGX5hG7q5aybNKlZqp8gn1O3/QM9d7io6kd58AgFDVYx0SK1SdHwzY 8kxeOMZHsz23rY6Hm4ifgtkHSQmjM29A8A5mPo5dWBga3w0dsXUPsQuu4gNp/1+IVJARg4t61X8 /i4liGZhL+PKhXkVRa9Vz3EivJaJeH24vg9zH142TVP2ASmBqO9thqQ6FRdFFHd7zDGWB9oBc23 lrEXthBkgheyE+gIs+ThwRrGsFkZ7H0wkqEzNAJIURd8eM9DwPOEvjRAb96aYTzRPNtI+USw X-Proofpoint-ORIG-GUID: 5YYDeGv5b81owpozaeAyIbJhu7F5KJ7H X-Proofpoint-GUID: 5YYDeGv5b81owpozaeAyIbJhu7F5KJ7H X-Authority-Analysis: v=2.4 cv=QfRmvtbv c=1 sm=1 tr=0 ts=686802af b=1 cx=c_pps a=WeWmnZmh0fydH62SvGsd2A==:117 a=WeWmnZmh0fydH62SvGsd2A==:17 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=wKuvFiaSGQ0qltdbU6+NXLB8nM8=:19 a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=Wb1JkmetP80A:10 a=GoEa3M9JfhUA:10 a=VwQbUJbxAAAA:8 a=yPCof4ZbAAAA:8 a=7RuVNZPScD8nBR_n4BoA:9 a=QEXdDO2ut3YA:10 On 25/06/2025 16:23, Antoine Tenart wrote: > Zero sized arrays do not go through the bitfield fixup logic, which in > turn makes them to skip the alignment infer logic. The result is > zero-length arrays being incorrectly aligned when an explicit statement > is needed. E.g. this happens for variable length arrays at end of > structures, or for 0-length ones. > > Before this patch we can see: > > struct skb_ext { > refcount_t refcnt; /* 0 4 */ > u8 offset[4]; /* 4 4 */ > u8 chunks; /* 8 1 */ > > /* XXX 7 bytes hole, try to pack */ > > char data[]; /* 16 0 */ > > /* size: 16, cachelines: 1, members: 4 */ > /* sum members: 9, holes: 1, sum holes: 7 */ > /* last cacheline: 16 bytes */ > } __attribute__((__aligned__(8))); > > offsetof(struct skb_ext, data) returns 9. > > After this patch we get: > > struct skb_ext { > refcount_t refcnt; /* 0 4 */ > u8 offset[4]; /* 4 4 */ > u8 chunks; /* 8 1 */ > > /* XXX 7 bytes hole, try to pack */ > > char data[] __attribute__((__aligned__(8))); /* 16 0 */ > > /* size: 16, cachelines: 1, members: 4 */ > /* sum members: 9, holes: 1, sum holes: 7 */ > /* forced alignments: 1, forced holes: 1, sum forced holes: 7 */ > /* last cacheline: 16 bytes */ > } __attribute__((__aligned__(8))); > > offsetof(struct skb_ext, data) returns 16. > > Signed-off-by: Antoine Tenart Reviewed-by: Alan Maguire with a small suggestion below.. > --- > btf_loader.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/btf_loader.c b/btf_loader.c > index f4f9f65289b5..76771afedd95 100644 > --- a/btf_loader.c > +++ b/btf_loader.c > @@ -648,6 +648,10 @@ static int class__fixup_btf_bitfields(const struct conf_load *conf, struct tag * > /* if BTF data is incorrect and has size == 0, skip field, > * instead of crashing */ nit: could we update the comment here to reflect the change, adding something like "however we still need to infer alignment for cases like zero/variable-length arays". > if (pos->byte_size == 0) { > + pos->alignment = class__infer_alignment(conf, > + pos->byte_offset, > + tag__natural_alignment(type, cu), > + smallest_offset); > continue; > } >