From: Yonghong Song > Sent: 15 February 2022 17:47 ... > > Let me try take a jab at it. Say here's a hypothetical BTF for a kernel > > module which only type information for `struct something *`: > > > > [5] PTR '(anon)' type_id=4 > > > > Which is built upon the follow base BTF: > > > > [1] INT 'unsigned char' size=1 bits_offset=0 nr_bits=8 encoding=(none) > > [2] PTR '(anon)' type_id=3 > > [3] STRUCT 'list_head' size=16 vlen=2 > > 'next' type_id=2 bits_offset=0 > > 'prev' type_id=2 bits_offset=64 > > [4] STRUCT 'something' size=2 vlen=2 > > 'locked' type_id=1 bits_offset=0 > > 'pending' type_id=1 bits_offset=8 > > > > Due to the situation mentioned in the beginning of the thread, the *runtime* > > kernel have a different base BTF, in this case type IDs are offset by 1 due > > to an additional typedef entry: > > > > [1] TYPEDEF 'u8' type_id=1 > > [2] INT 'unsigned char' size=1 bits_offset=0 nr_bits=8 encoding=(none) > > [3] PTR '(anon)' type_id=3 > > [4] STRUCT 'list_head' size=16 vlen=2 > > 'next' type_id=2 bits_offset=0 > > 'prev' type_id=2 bits_offset=64 > > [5] STRUCT 'something' size=2 vlen=2 > > 'locked' type_id=1 bits_offset=0 > > 'pending' type_id=1 bits_offset=8 > > > > Then when loading the BTF on kernel module on the runtime, the kernel will > > mistakenly interprets "PTR '(anon)' type_id=4" as `struct list_head *` > > rather than `struct something *`. > > > > Does this should possible? (at least theoretically) > > Thanks for explanation. Yes, from BTF type resolution point of view, > yes it is possible. This looks so much like the old 'shared library function number' ordinals from pre-SYSV and early windows shared libraries. There is a good reason why it isn't done that way any more. Has someone re-invented the square wheel?? David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
