On Tue, 2025-07-08 at 22:58 -0700, Eduard Zingerman wrote:
[...]
> This seems to work:
> https://github.com/eddyz87/llvm-project/tree/separate-jumptables-section.1
Pushed an update:
- correct offsets computation;
- avoid relocations in the .jumptables section.
Here is how it looks now (updated session.log attached):
foo: # @foo
# %bb.0: # %entry
w1 = *(u32 *)(r1 + 0)
if w1 > 31 goto LBB0_2
# %bb.1: # %entry
.LJTI0_0:
.reloc 0, FK_SecRel_8, .BPF.JT.0.0
gotox r1
goto LBB0_2
LBB0_7:
w1 = 2
goto LBB0_3
...
.Lfunc_end0:
.size foo, .Lfunc_end0-foo
.section .jumptables,"",@progbits
.L0_0_set_7 = LBB0_7-.LJTI0_0
.L0_0_set_2 = LBB0_2-.LJTI0_0
.L0_0_set_8 = LBB0_8-.LJTI0_0
.L0_0_set_9 = LBB0_9-.LJTI0_0
.L0_0_set_10 = LBB0_10-.LJTI0_0
.BPF.JT.0.0:
.long .L0_0_set_7
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
...
I think this is a correct form, further changes should be LLVM
internal.
$ cat jump-table-test.c
int bar(int v);
int foo(struct simple_ctx *ctx)
{
int ret_user;
switch (ctx->x) {
case 0:
ret_user = 2;
break;
case 11:
ret_user = 3;
break;
case 27:
ret_user = 4;
break;
case 31:
ret_user = 5;
break;
default:
ret_user = 19;
break;
}
switch (bar(ret_user)) {
case 1:
ret_user = 5;
break;
case 12:
ret_user = 7;
break;
case 27:
ret_user = 23;
break;
case 32:
ret_user = 37;
break;
case 44:
ret_user = 77;
break;
default:
ret_user = 11;
break;
}
return ret_user;
}
$ clang --target=bpf -O2 -S -o - jump-table-test.c
.text
.globl foo # -- Begin function foo
.p2align 3
.type foo,@function
foo: # @foo
# %bb.0: # %entry
w1 = *(u32 *)(r1 + 0)
if w1 > 31 goto LBB0_2
# %bb.1: # %entry
.LJTI0_0:
.reloc 0, FK_SecRel_8, .BPF.JT.0.0
gotox r1
goto LBB0_2
LBB0_7:
w1 = 2
goto LBB0_3
LBB0_9: # %sw.bb2
w1 = 4
goto LBB0_3
LBB0_8: # %sw.bb1
w1 = 3
goto LBB0_3
LBB0_10: # %sw.bb3
w1 = 5
goto LBB0_3
LBB0_2: # %sw.default
w1 = 19
LBB0_3: # %sw.epilog
call bar
# kill: def $w0 killed $w0 def $r0
w0 += -1
if w0 > 43 goto LBB0_5
# %bb.4: # %sw.epilog
.LJTI0_1:
.reloc 0, FK_SecRel_8, .BPF.JT.0.1
gotox r0
goto LBB0_5
LBB0_11:
w0 = 5
goto LBB0_6
LBB0_5: # %sw.default9
w0 = 11
goto LBB0_6
LBB0_13: # %sw.bb6
w0 = 23
goto LBB0_6
LBB0_12: # %sw.bb5
w0 = 7
goto LBB0_6
LBB0_14: # %sw.bb7
w0 = 37
goto LBB0_6
LBB0_15: # %sw.bb8
w0 = 77
LBB0_6: # %sw.epilog10
exit
.Lfunc_end0:
.size foo, .Lfunc_end0-foo
.section .jumptables,"",@progbits
.L0_0_set_7 = LBB0_7-.LJTI0_0
.L0_0_set_2 = LBB0_2-.LJTI0_0
.L0_0_set_8 = LBB0_8-.LJTI0_0
.L0_0_set_9 = LBB0_9-.LJTI0_0
.L0_0_set_10 = LBB0_10-.LJTI0_0
.BPF.JT.0.0:
.long .L0_0_set_7
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_8
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_9
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_2
.long .L0_0_set_10
.size .BPF.JT.0.0, 128
.L0_1_set_11 = LBB0_11-.LJTI0_1
.L0_1_set_5 = LBB0_5-.LJTI0_1
.L0_1_set_12 = LBB0_12-.LJTI0_1
.L0_1_set_13 = LBB0_13-.LJTI0_1
.L0_1_set_14 = LBB0_14-.LJTI0_1
.L0_1_set_15 = LBB0_15-.LJTI0_1
.BPF.JT.0.1:
.long .L0_1_set_11
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_12
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_13
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_14
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_5
.long .L0_1_set_15
.size .BPF.JT.0.1, 176
# -- End function
.addrsig
$ clang --target=bpf -O2 -c -o jump-table-test.o jump-table-test.c
$ llvm-readelf -r --symbols --sections jump-table-test.o
Section Headers:
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000 00 0 0 0
[ 1] .strtab STRTAB 0000000000000000 000320 000067 00 0 0 1
[ 2] .text PROGBITS 0000000000000000 000040 0000f0 00 AX 0 0 8
[ 3] .rel.text REL 0000000000000000 0002f0 000030 10 I 6 2 8
[ 4] .jumptables PROGBITS 0000000000000000 000130 000130 00 0 0 1
[ 5] .llvm_addrsig LLVM_ADDRSIG 0000000000000000 000320 000000 00 E 6 0 1
[ 6] .symtab SYMTAB 0000000000000000 000260 000090 18 1 2 8
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
R (retain), p (processor specific)
Relocation section '.rel.text' at offset 0x2f0 contains 3 entries:
Offset Info Type Symbol's Value Symbol's Name
0000000000000010 0000000300000001 R_BPF_64_64 0000000000000000 .BPF.JT.0.0
0000000000000068 000000040000000a R_BPF_64_32 0000000000000000 bar
0000000000000080 0000000500000001 R_BPF_64_64 0000000000000080 .BPF.JT.0.1
Symbol table '.symtab' contains 6 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS jump-table-test.c
2: 0000000000000000 240 FUNC GLOBAL DEFAULT 2 foo
3: 0000000000000000 128 NOTYPE GLOBAL DEFAULT 4 .BPF.JT.0.0
4: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND bar
5: 0000000000000080 176 NOTYPE GLOBAL DEFAULT 4 .BPF.JT.0.1
$ llvm-objdump --no-show-raw-insn -Sdr jump-table-test.o
jump-table-test.o: file format elf64-bpf
Disassembly of section .text:
0000000000000000 <foo>:
0: w1 = *(u32 *)(r1 + 0x0)
1: if w1 > 0x1f goto +0xa <foo+0x60>
2: gotox r1
0000000000000010: R_BPF_64_64 .BPF.JT.0.0
3: goto +0x8 <foo+0x60>
4: w1 = 0x2
5: goto +0x7 <foo+0x68>
6: w1 = 0x4
7: goto +0x5 <foo+0x68>
8: w1 = 0x3
9: goto +0x3 <foo+0x68>
10: w1 = 0x5
11: goto +0x1 <foo+0x68>
12: w1 = 0x13
13: call -0x1
0000000000000068: R_BPF_64_32 bar
14: w0 += -0x1
15: if w0 > 0x2b goto +0x4 <foo+0xa0>
16: gotox r0
0000000000000080: R_BPF_64_64 .BPF.JT.0.1
17: goto +0x2 <foo+0xa0>
18: w0 = 0x5
19: goto +0x9 <foo+0xe8>
20: w0 = 0xb
21: goto +0x7 <foo+0xe8>
22: w0 = 0x17
23: goto +0x5 <foo+0xe8>
24: w0 = 0x7
25: goto +0x3 <foo+0xe8>
26: w0 = 0x25
27: goto +0x1 <foo+0xe8>
28: w0 = 0x4d
29: exit
