[BPF] Avoid relocation for jumptable entries #166301
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cc @aspsk |
Currently, the jump table entry contains labels only.
For example, the following is one example:
BPF.JT.0.0:
.quad LBB0_1
.quad LBB0_2
.size BPF.JT.0.0, 16
Since the jump table entry contains a label, the relocation
is necessary so linker can resolve the label value. The relocation
looks like below:
Relocation section '.rel.jumptables' at offset 0x160 contains 2 entries:
Offset Info Type Symbol's Value Symbol's Name
0000000000000000 0000000200000002 R_BPF_64_ABS64 0000000000000000 .text
0000000000000008 0000000200000002 R_BPF_64_ABS64 0000000000000000 .text
You can see that the symbol value is 0 which makes .rel.jumptables
not very useful.
Instead of having the label itself in the jump table entry,
use the difference of label and the section begin symbol.
This can avoid the relocation and the eventual jumptable
entries in object file remains the same as before.
Hex dump of section '.jumptables':
0x00000000 68000000 00000000 78000000 00000000 h.......x.......
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@eddyz87 has similar idea before (to have (label - <start_section>)/8 as the jump table entry). Later on it is decided that label itself is good enough. This patch merely change the jump table entry to (label - <start_section>) to avoid unnecessary relocation for jump table. |
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Thanks @yonghong-song, tested as well |
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Currently, the jump table entry contains labels only. For example, the following is one example:
Since the jump table entry contains a label, the relocation is necessary so linker can resolve the label value. The relocation looks like below:
You can see that the symbol value is 0 which makes .rel.jumptables not very useful.
Instead of having the label itself in the jump table entry, use the difference of label and the section begin symbol. This can avoid the relocation and the eventual jumptable entries in object file remains the same as before.