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Daniel Hodges reported a jit error when playing with a sched-ext program.
The error message is:
unexpected jmp_cond padding: -4 bytes
But further investigation shows the error is actual due to failed
convergence. The following are some analysis:
...
pass4, final_proglen=4391:
...
20e: 48 85 ff test rdi,rdi
211: 74 7d je 0x290
213: 48 8b 77 00 mov rsi,QWORD PTR [rdi+0x0]
...
289: 48 85 ff test rdi,rdi
28c: 74 17 je 0x2a5
28e: e9 7f ff ff ff jmp 0x212
293: bf 03 00 00 00 mov edi,0x3
Note that insn at 0x211 is 2-byte cond jump insn for offset 0x7d (-125)
and insn at 0x28e is 5-byte jmp insn with offset -129.
pass5, final_proglen=4392:
...
20e: 48 85 ff test rdi,rdi
211: 0f 84 80 00 00 00 je 0x297
217: 48 8b 77 00 mov rsi,QWORD PTR [rdi+0x0]
...
28d: 48 85 ff test rdi,rdi
290: 74 1a je 0x2ac
292: eb 84 jmp 0x218
294: bf 03 00 00 00 mov edi,0x3
Note that insn at 0x211 is 6-byte cond jump insn now since its offset
becomes 0x80 based on previous round (0x293 - 0x213 = 0x80). At the same
time, insn at 0x292 is a 2-byte insn since its offset is -124.
pass6 will repeat the same code as in pass4. pass7 will repeat the same
code as in pass5, and so on. This will prevent eventual convergence.
Passes 1-14 are with padding = 0. At pass15, padding is 1 and related
insn looks like:
211: 0f 84 80 00 00 00 je 0x297
217: 48 8b 77 00 mov rsi,QWORD PTR [rdi+0x0]
...
24d: 48 85 d2 test rdx,rdx
The similar code in pass14:
211: 74 7d je 0x290
213: 48 8b 77 00 mov rsi,QWORD PTR [rdi+0x0]
...
249: 48 85 d2 test rdx,rdx
24c: 74 21 je 0x26f
24e: 48 01 f7 add rdi,rsi
...
Before generating the following insn,
250: 74 21 je 0x273
"padding = 1" enables some checking to ensure nops is either 0 or 4
where
#define INSN_SZ_DIFF (((addrs[i] - addrs[i - 1]) - (prog - temp)))
nops = INSN_SZ_DIFF - 2
In this specific case,
addrs[i] = 0x24e // from pass14
addrs[i-1] = 0x24d // from pass15
prog - temp = 3 // from 'test rdx,rdx' in pass15
so
nops = -4
and this triggers the failure.
To fix the issue, we need to break cycles of je <-> jmp. For example,
in the above case, we have
211: 74 7d je 0x290
the offset is 0x7d. If 2-byte je insn is generated only if
the offset is less than 0x7d (<= 0x7c), the cycle can be
break and we can achieve the convergence.
I did some study on other cases like je <-> je, jmp <-> je and
jmp <-> jmp which may cause cycles. Those cases are not from actual
reproducible cases since it is pretty hard to construct a test case
for them. the results show that the offset <= 0x7b (0x7b = 123) should
be enough to cover all cases. This patch added a new helper to generate 8-bit
cond/uncond jmp insns only if the offset range is [-128, 123].
Reported-by: Daniel Hodges <hodgesd@meta.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240904221251.37109-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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