On Wed, Sep 01, 2021 at 01:31:18PM +0200, Greg KH wrote:

On Wed, Sep 01, 2021 at 01:30:37PM +0200, Greg KH wrote:

quoted

On Mon, Aug 30, 2021 at 03:32:08PM -0300, Thadeu Lima de Souza Cascardo wrote:

quoted

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 <redacted>
---
 kernel/bpf/core.c | 32 +++++++++++++++-----------------
 1 file changed, 15 insertions(+), 17 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index e7211b0fa27c..30d871be9974 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c

@@ -616,9 +616,6 @@ 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!

@@ -951,6 +948,7 @@ static unsigned int ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn,
 	u32 tail_call_cnt = 0;
 	void *ptr;
 	int off;
+	u64 tmp;
 
 #define CONT	 ({ insn++; goto select_insn; })
 #define CONT_JMP ({ insn++; goto select_insn; })

@@ -1013,22 +1011,22 @@ 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))

@@ -1038,17 +1036,17 @@ 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) {

-- 
2.30.2

Oops, no, this patch causes build errors:

kernel/bpf/core.c: In function ‘___bpf_prog_run’:
kernel/bpf/core.c:951:13: error: redeclaration of ‘tmp’ with no linkage
  951 |         u64 tmp;
      |             ^~~
kernel/bpf/core.c:839:13: note: previous declaration of ‘tmp’ with type ‘u64’ {aka ‘long long unsigned int’}
  839 |         u64 tmp;
      |             ^~~
make[2]: *** [scripts/Makefile.build:329: kernel/bpf/core.o] Error 1


Please fix up and resend the whole series, as I will go drop these 3
patches from the 4.14.y queue now.

All _4_ patches I mean.  now dropped...

Ah... it seems I only built it with CONFIG_BPF_JIT_ALWAYS_ON. I will build with
both that option on and off and check the results.

Thanks for catching this.

Cascardo.