bpf: move clearing of A/X into classic to eBPF migration prologue

Back in the days where eBPF (or back then "internal BPF" ;->) was not
exposed to user space, and only the classic BPF programs internally
translated into eBPF programs, we missed the fact that for classic BPF
A and X needed to be cleared. It was fixed back then via 83d5b7ef99c9
("net: filter: initialize A and X registers"), and thus classic BPF
specifics were added to the eBPF interpreter core to work around it.

This added some confusion for JIT developers later on that take the
eBPF interpreter code as an example for deriving their JIT. F.e. in
f75298f5c3fe ("s390/bpf: clear correct BPF accumulator register"), at
least X could leak stack memory. Furthermore, since this is only needed
for classic BPF translations and not for eBPF (verifier takes care
that read access to regs cannot be done uninitialized), more complexity
is added to JITs as they need to determine whether they deal with
migrations or native eBPF where they can just omit clearing A/X in
their prologue and thus reduce image size a bit, see f.e. cde66c2d88da
("s390/bpf: Only clear A and X for converted BPF programs"). In other
cases (x86, arm64), A and X is being cleared in the prologue also for
eBPF case, which is unnecessary.

Lets move this into the BPF migration in bpf_convert_filter() where it
actually belongs as long as the number of eBPF JITs are still few. It
can thus be done generically; allowing us to remove the quirk from
__bpf_prog_run() and to slightly reduce JIT image size in case of eBPF,
while reducing code duplication on this matter in current(/future) eBPF
JITs.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Tested-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Cc: Zi Shen Lim <zlim.lnx@gmail.com>
Cc: Yang Shi <yang.shi@linaro.org>
Acked-by: Yang Shi <yang.shi@linaro.org>
Acked-by: Zi Shen Lim <zlim.lnx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 11 deletions

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index 9a0c4c22e536..3c0bfc1f2694 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -408,7 +408,7 @@ static void emit_load_skb_data_hlen(struct bpf_jit *jit)
  * Save registers and create stack frame if necessary.
  * See stack frame layout desription in "bpf_jit.h"!
  */
-static void bpf_jit_prologue(struct bpf_jit *jit, bool is_classic)
+static void bpf_jit_prologue(struct bpf_jit *jit)
 {
         if (jit->seen & SEEN_TAIL_CALL) {
                 /* xc STK_OFF_TCCNT(4,%r15),STK_OFF_TCCNT(%r15) */
@@ -448,15 +448,6 @@ static void bpf_jit_prologue(struct bpf_jit *jit, bool is_classic)
                 /* stg %b1,ST_OFF_SKBP(%r0,%r15) */
                 EMIT6_DISP_LH(0xe3000000, 0x0024, REG_W1, REG_0, REG_15,
                               STK_OFF_SKBP);
-        /* Clear A (%b0) and X (%b7) registers for converted BPF programs */
-        if (is_classic) {
-                if (REG_SEEN(BPF_REG_A))
-                        /* lghi %ba,0 */
-                        EMIT4_IMM(0xa7090000, BPF_REG_A, 0);
-                if (REG_SEEN(BPF_REG_X))
-                        /* lghi %bx,0 */
-                        EMIT4_IMM(0xa7090000, BPF_REG_X, 0);
-        }
 }
 
 /*
@@ -1245,7 +1236,7 @@ static int bpf_jit_prog(struct bpf_jit *jit, struct bpf_prog *fp)
         jit->lit = jit->lit_start;
         jit->prg = 0;
 
-        bpf_jit_prologue(jit, bpf_prog_was_classic(fp));
+        bpf_jit_prologue(jit);
         for (i = 0; i < fp->len; i += insn_count) {
                 insn_count = bpf_jit_insn(jit, fp, i);
                 if (insn_count < 0)