[PATCH bpf] s390/bpf: use 32-bit index for tail calls

[PATCH bpf] s390/bpf: use 32-bit index for tail calls

[Ilya Leoshkevich]

"p runtime/jit: pass > 32bit index to tail_call" fails when
bpf_jit_enable=1, because the tail call is not executed.

This in turn is because the generated code assumes index is 64-bit,
while it must be 32-bit, and as a result prog array bounds check fails,
while it should pass. Even if bounds check would have passed, the code
that follows uses 64-bit index to compute prog array offset.

Fix by using clrj instead of clgrj for comparing index with array size,
and also by using llgfr for truncating index to 32 bits before using it
to compute prog array offset.

Fixes: 6651ee070b31 ("s390/bpf: implement bpf_tail_call() helper")
Reported-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 arch/s390/net/bpf_jit_comp.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index 6299156f9738..955eb355c2fd 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -1049,8 +1049,8 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp, int i
 		/* llgf %w1,map.max_entries(%b2) */
 		EMIT6_DISP_LH(0xe3000000, 0x0016, REG_W1, REG_0, BPF_REG_2,
 			      offsetof(struct bpf_array, map.max_entries));
-		/* clgrj %b3,%w1,0xa,label0: if %b3 >= %w1 goto out */
-		EMIT6_PCREL_LABEL(0xec000000, 0x0065, BPF_REG_3,
+		/* clrj %b3,%w1,0xa,label0: if (u32)%b3 >= (u32)%w1 goto out */
+		EMIT6_PCREL_LABEL(0xec000000, 0x0077, BPF_REG_3,
 				  REG_W1, 0, 0xa);
 
 		/*
@@ -1076,8 +1076,10 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp, int i
 		 *         goto out;
 		 */
 
-		/* sllg %r1,%b3,3: %r1 = index * 8 */
-		EMIT6_DISP_LH(0xeb000000, 0x000d, REG_1, BPF_REG_3, REG_0, 3);
+		/* llgfr %r1,%b3: %r1 = (u32) index */
+		EMIT4(0xb9160000, REG_1, BPF_REG_3);
+		/* sllg %r1,%r1,3: %r1 *= 8 */
+		EMIT6_DISP_LH(0xeb000000, 0x000d, REG_1, REG_1, REG_0, 3);
 		/* lg %r1,prog(%b2,%r1) */
 		EMIT6_DISP_LH(0xe3000000, 0x0004, REG_1, BPF_REG_2,
 			      REG_1, offsetof(struct bpf_array, ptrs));
-- 
2.21.0

Re: [PATCH bpf] s390/bpf: use 32-bit index for tail calls

[Daniel Borkmann]

On 8/12/19 6:18 PM, Ilya Leoshkevich wrote:
> "p runtime/jit: pass > 32bit index to tail_call" fails when
> bpf_jit_enable=1, because the tail call is not executed.
> 
> This in turn is because the generated code assumes index is 64-bit,
> while it must be 32-bit, and as a result prog array bounds check fails,
> while it should pass. Even if bounds check would have passed, the code
> that follows uses 64-bit index to compute prog array offset.
> 
> Fix by using clrj instead of clgrj for comparing index with array size,
> and also by using llgfr for truncating index to 32 bits before using it
> to compute prog array offset.
> 
> Fixes: 6651ee070b31 ("s390/bpf: implement bpf_tail_call() helper")
> Reported-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
> Acked-by: Vasily Gorbik <gor@linux.ibm.com>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

Applied, thanks!