[PATCH 4.14 v2 1/4] bpf: Do not use ax register in interpreter on div/mod

Edward Liaw
2023-02-22T19:29:46Z

From: Daniel Borkmann <daniel@iogearbox.net>

Partially undo old commit 144cd91c4c2b ("bpf: move tmp variable into ax
register in interpreter"). The reason we need this here is because ax
register will be used for holding temporary state for div/mod instruction
which otherwise interpreter would corrupt. This will cause a small +8 byte
stack increase for interpreter, but with the gain that we can use it from
verifier rewrites as scratch register.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
[cascardo: This partial revert is needed in order to support using AX for
the following two commits, as there is no JMP32 on 4.19.y]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
[edliaw: Removed redeclaration of tmp]
Signed-off-by: Edward Liaw <edliaw@google.com>
---
kernel/bpf/core.c | 31 ++++++++++++++-----------------
1 file changed, 14 insertions(+), 17 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 5d649983de07..4ddb846693bb 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -663,9 +663,6 @@ static int bpf_jit_blind_insn(const struct bpf_insn *from,
* below.
*
* Constant blinding is only used by JITs, not in the interpreter.
- * The interpreter uses AX in some occasions as a local temporary
- * register e.g. in DIV or MOD instructions.
- *
* In restricted circumstances, the verifier can also use the AX
* register for rewrites as long as they do not interfere with
* the above cases!
@@ -1060,22 +1057,22 @@ static unsigned int ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn,
ALU64_MOD_X:
if (unlikely(SRC == 0))
return 0;
- div64_u64_rem(DST, SRC, &AX);
- DST = AX;
+ div64_u64_rem(DST, SRC, &tmp);
+ DST = tmp;
CONT;
ALU_MOD_X:
if (unlikely((u32)SRC == 0))
return 0;
- AX = (u32) DST;
- DST = do_div(AX, (u32) SRC);
+ tmp = (u32) DST;
+ DST = do_div(tmp, (u32) SRC);
CONT;
ALU64_MOD_K:
- div64_u64_rem(DST, IMM, &AX);
- DST = AX;
+ div64_u64_rem(DST, IMM, &tmp);
+ DST = tmp;
CONT;
ALU_MOD_K:
- AX = (u32) DST;
- DST = do_div(AX, (u32) IMM);
+ tmp = (u32) DST;
+ DST = do_div(tmp, (u32) IMM);
CONT;
ALU64_DIV_X:
if (unlikely(SRC == 0))
@@ -1085,17 +1082,17 @@ static unsigned int ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn,
ALU_DIV_X:
if (unlikely((u32)SRC == 0))
return 0;
- AX = (u32) DST;
- do_div(AX, (u32) SRC);
- DST = (u32) AX;
+ tmp = (u32) DST;
+ do_div(tmp, (u32) SRC);
+ DST = (u32) tmp;
CONT;
ALU64_DIV_K:
DST = div64_u64(DST, IMM);
CONT;
ALU_DIV_K:
- AX = (u32) DST;
- do_div(AX, (u32) IMM);
- DST = (u32) AX;
+ tmp = (u32) DST;
+ do_div(tmp, (u32) IMM);
+ DST = (u32) tmp;
CONT;
ALU_END_TO_BE:
switch (IMM) {
--
2.39.2.637.g21b0678d19-goog