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Subject[PATCH 4.14 78/92] bpf: move tmp variable into ax register in interpreter
Date
From: Daniel Borkmann <daniel@iogearbox.net>

commit 144cd91c4c2bced6eb8a7e25e590f6618a11e854 upstream.

This change moves the on-stack 64 bit tmp variable in ___bpf_prog_run()
into the hidden ax register. The latter is currently only used in JITs
for constant blinding as a temporary scratch register, meaning the BPF
interpreter will never see the use of ax. Therefore it is safe to use
it for the cases where tmp has been used earlier. This is needed to later
on allow restricted hidden use of ax in both interpreter and JITs.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
[backported to 4.14 sblbir]
Signed-off-by: Balbir Singh <sblbir@amzn.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/filter.h | 3 ++-
kernel/bpf/core.c | 31 ++++++++++++++++---------------
2 files changed, 18 insertions(+), 16 deletions(-)

--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -53,7 +53,8 @@ struct bpf_prog_aux;
* constants. See JIT pre-step in bpf_jit_blind_constants().
*/
#define BPF_REG_AX MAX_BPF_REG
-#define MAX_BPF_JIT_REG (MAX_BPF_REG + 1)
+#define MAX_BPF_EXT_REG (MAX_BPF_REG + 1)
+#define MAX_BPF_JIT_REG MAX_BPF_EXT_REG

/* unused opcode to mark special call to bpf_tail_call() helper */
#define BPF_TAIL_CALL 0xf0
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -51,6 +51,7 @@
#define DST regs[insn->dst_reg]
#define SRC regs[insn->src_reg]
#define FP regs[BPF_REG_FP]
+#define AX regs[BPF_REG_AX]
#define ARG1 regs[BPF_REG_ARG1]
#define CTX regs[BPF_REG_CTX]
#define IMM insn->imm
@@ -939,22 +940,22 @@ select_insn:
ALU64_MOD_X:
if (unlikely(SRC == 0))
return 0;
- div64_u64_rem(DST, SRC, &tmp);
- DST = tmp;
+ div64_u64_rem(DST, SRC, &AX);
+ DST = AX;
CONT;
ALU_MOD_X:
if (unlikely((u32)SRC == 0))
return 0;
- tmp = (u32) DST;
- DST = do_div(tmp, (u32) SRC);
+ AX = (u32) DST;
+ DST = do_div(AX, (u32) SRC);
CONT;
ALU64_MOD_K:
- div64_u64_rem(DST, IMM, &tmp);
- DST = tmp;
+ div64_u64_rem(DST, IMM, &AX);
+ DST = AX;
CONT;
ALU_MOD_K:
- tmp = (u32) DST;
- DST = do_div(tmp, (u32) IMM);
+ AX = (u32) DST;
+ DST = do_div(AX, (u32) IMM);
CONT;
ALU64_DIV_X:
if (unlikely(SRC == 0))
@@ -964,17 +965,17 @@ select_insn:
ALU_DIV_X:
if (unlikely((u32)SRC == 0))
return 0;
- tmp = (u32) DST;
- do_div(tmp, (u32) SRC);
- DST = (u32) tmp;
+ AX = (u32) DST;
+ do_div(AX, (u32) SRC);
+ DST = (u32) AX;
CONT;
ALU64_DIV_K:
DST = div64_u64(DST, IMM);
CONT;
ALU_DIV_K:
- tmp = (u32) DST;
- do_div(tmp, (u32) IMM);
- DST = (u32) tmp;
+ AX = (u32) DST;
+ do_div(AX, (u32) IMM);
+ DST = (u32) AX;
CONT;
ALU_END_TO_BE:
switch (IMM) {
@@ -1278,7 +1279,7 @@ STACK_FRAME_NON_STANDARD(___bpf_prog_run
static unsigned int PROG_NAME(stack_size)(const void *ctx, const struct bpf_insn *insn) \
{ \
u64 stack[stack_size / sizeof(u64)]; \
- u64 regs[MAX_BPF_REG]; \
+ u64 regs[MAX_BPF_EXT_REG]; \
\
FP = (u64) (unsigned long) &stack[ARRAY_SIZE(stack)]; \
ARG1 = (u64) (unsigned long) ctx; \

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