Messages in this thread | | | Subject | Re: BTF_KIND_FWD enums | From | Yonghong Song <> | Date | Wed, 29 Jul 2020 00:09:40 -0700 |
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On 7/28/20 1:56 PM, Giuliano Procida wrote: > Hi. > > Re: https://github.com/torvalds/linux/commit/9d5f9f701b1891466fb3dbb1806ad97716f95cc3 > > Both GCC and LLVM support forward-declared (a.k.a. incomplete) enums > as a language extension - > https://gcc.gnu.org/onlinedocs/gcc/Incomplete-Enums.html. > > (C++11 has a different notion of incomplete enum type - opaque enum > declaration - storage size is known but enumerators are not) > > Forward-declared enums feature in various places in kernel code and > allow the usual things to be done (passing around pointers to such). > I'm curious as to if and how they are they are handled by BTF and > whether a further change to btf_type is needed: > > 1. Use BTF_KIND_FWD, with another spare bit to allow up to 4 kinds of > forward-declaration; or > 2. use BTF_KIND_ENUM, kind_flag 0 and vlen 0 (as empty enums are > currently illegal C); or
#2 above is the current way. -bash-4.4$ cat t.c enum H; enum H * func(enum H *arg) { return arg; } -bash-4.4$ clang -target bpf -S -g -O2 t.c
-bash-4.4$
The generated BTF enum type:
.long 1 # BTF_KIND_ENUM(id = 2) .long 100663296 # 0x6000000 .long 0
So vlen = 0 here indicates it is a forward declaration.
> 3. use BTF_KIND_ENUM, kind_flag 1 and vlen 0. > > If I had a working pahole -J, I'd test this myself. :-) > > $ cat /tmp/en.c > enum H; > enum H * fun(enum H * x) { return x; } > $ clang -Wall -Wextra -ggdb -c /tmp/en.c > $ build/pahole -J /tmp/en.o > Segmentation fault > $ build/pahole -J /dev/null > btf_elf__new: cannot get elf header. > ctf__new: cannot get elf header. > Segmentation fault
-bash-4.4$ cat t.c enum H; enum H * func(enum H *arg) { return arg; } -bash-4.4$ ~/work/pahole/build/pahole --version v1.17 -bash-4.4$ clang -Wall -Wextra -ggdb -c t.c
-bash-4.4$ ~/work/pahole/build/pahole -JV t.o File t.o: [1] PTR (anon) type_id=2 [2] ENUM H size=4 vlen=0 [3] INT (anon) size=4 bit_offset=0 nr_bits=32 encoding=(none) [4] FUNC_PROTO (anon) return=1 args=(1 arg) [5] FUNC func type_id=4
pahole also generates vlen=0 ENUM type to indicate it is a forward declaration.
Maybe your pahole is too old?
> > My interest here is that I helped add support for incomplete enums to > libabigail which we're using to monitor kernel ABIs. > > Regards, > Giuliano. > > (resend due to email address typo) >
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