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

[Edward Liaw]

From: Daniel Borkmann <daniel@xxxxxxxxxxxxx>

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@xxxxxxxxxxxxx>
Reviewed-by: John Fastabend <john.fastabend@xxxxxxxxx>
[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@xxxxxxxxxxxxx>
[edliaw: Removed redeclaration of tmp]
Signed-off-by: Edward Liaw <edliaw@xxxxxxxxxx>
---
 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