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Andrii reported an issue with optimized uprobes [1] that can clobber
redzone area with call instruction storing return address on stack
where user code may keep temporary data without adjusting rsp.
Fixing this by moving the optimized uprobes on top of 10-bytes nop
instruction, so we can squeeze another instruction to escape the
redzone area before doing the call, like:
lea -0x80(%rsp), %rsp
call tramp
Note the lea instruction is used to adjust the rsp register without
changing the flags.
We use nop10 and following transformation to optimized instructions
above and back as suggested by Peterz [2].
Optimize path (int3_update_optimize):
1) Initial state after set_swbp() installed the uprobe:
cc 2e 0f 1f 84 00 00 00 00 00
From offset 0 this is INT3 followed by the tail of the original
10-byte NOP.
After a previous unoptimization bytes 5..9 may still contain the
old call instruction, which remains valid for threads already there.
2) Rewrite the LEA tail and call displacement:
cc [8d 64 24 80 e8 d0 d1 d2 d3]
From offset 0 this traps on the uprobe INT3. Bytes 1..9 are not
executable entry points while byte 0 is trapped.
3) Publish the first LEA byte:
[48] 8d 64 24 80 e8 d0 d1 d2 d3
From offset 0 this is:
lea -0x80(%rsp), %rsp
call <uprobe-trampoline>
Unoptimize path (int3_update_unoptimize):
1) Initial optimized state:
48 8d 64 24 80 e8 d0 d1 d2 d3
Same as 3) above.
2) Trap new entries before restoring the NOP bytes:
[cc] 8d 64 24 80 e8 d0 d1 d2 d3
From offset 0 this traps. A thread that had already executed the
LEA can still reach the intact CALL at offset 5.
3) Restore bytes 1..4 of the original NOP while keeping byte 0 trapped
and byte 5 as CALL.
cc [2e 0f 1f 84] e8 d0 d1 d2 d3
From offset 0 this still traps. Offset 5 is still the CALL for any
thread that was already past the first LEA byte.
4) Publish the first byte of the original NOP:
[66] 2e 0f 1f 84 e8 d0 d1 d2 d3
From offset 0 this is the restored 10-byte NOP; the CALL opcode and
displacement are now only NOP operands. Offset 5 still decodes as
CALL for a thread that was already there.
Tthere is only a single target uprobe-trampoline for the given nop10
instruction address, so the CALL instruction will not be changed across
unoptimization/optimization cycles.
Therefore, any task that is preempted at the CALL instruction is guaranteed
to observe that CALL and not anything else.
Note as explained in [2] we need to use following nop10:
PF1 PF2 ESC NOPL MOD SIB DISP32
NOP10: 0x66, 0x2e, 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00 -- cs nopw 0x00000000(%rax,%rax,1)
which means we need to allow 0x2e prefix which maps to INAT_PFX_CS
attribute in is_prefix_bad function.
Also changing the uprobe syscall error when called out of uprobe
trampoline to -EPROTO, so we are able to detect the fixed kernel.
The optimized uprobe performance stays the same:
uprobe-nop : 3.129 ± 0.013M/s
uprobe-push : 3.045 ± 0.006M/s
uprobe-ret : 1.095 ± 0.004M/s
--> uprobe-nop10 : 7.170 ± 0.020M/s
uretprobe-nop : 2.143 ± 0.021M/s
uretprobe-push : 2.090 ± 0.000M/s
uretprobe-ret : 0.942 ± 0.000M/s
--> uretprobe-nop10: 3.381 ± 0.003M/s
usdt-nop : 3.245 ± 0.004M/s
--> usdt-nop10 : 7.256 ± 0.023M/s
[1] https://lore.kernel.org/bpf/20260509003146.976844-1-andrii@kernel.org/
[2] https://lore.kernel.org/bpf/20260518104306.GU3102624@noisy.programming.kicks-ass.net/#t
Closes: https://lore.kernel.org/bpf/20260509003146.976844-1-andrii@kernel.org/
Fixes: ba2bfc97b4 ("uprobes/x86: Add support to optimize uprobes")
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Assisted-by: Codex:GPT-5.5
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Link: https://patch.msgid.link/20260703114917.238144-6-jolsa@kernel.org
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