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libbpf: support llvm-generated indirect jumps

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For v4 instruction set LLVM is allowed to generate indirect jumps for
switch statements and for 'goto *rX' assembly. Every such a jump will
be accompanied by necessary metadata, e.g. (`llvm-objdump -Sr ...`):

       0:       r2 = 0x0 ll
                0000000000000030:  R_BPF_64_64  BPF.JT.0.0

Here BPF.JT.1.0 is a symbol residing in the .jumptables section:

    Symbol table:
       4: 0000000000000000   240 OBJECT  GLOBAL DEFAULT     4 BPF.JT.0.0

The -bpf-min-jump-table-entries llvm option may be used to control the
minimal size of a switch which will be converted to an indirect jumps.

Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251105090410.1250500-11-a.s.protopopov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Anton Protopopov
2025-11-05 09:04:08 +00:00
committed by Andrii Nakryiko
parent 349b78117b
commit 24a89cb35d
3 changed files with 251 additions and 1 deletions
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2
src/libbpf_internal.h
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@@ -74,6 +74,8 @@
#define ELF64_ST_VISIBILITY(o) ((o) & 0x03)
#endif
#define JUMPTABLES_SEC ".jumptables"
#define BTF_INFO_ENC(kind, kind_flag, vlen) \
((!!(kind_flag) << 31) | ((kind) << 24) | ((vlen) & BTF_MAX_VLEN))
#define BTF_TYPE_ENC(name, info, size_or_type) (name), (info), (size_or_type)