[PATCH 0/5] s390/bpf: fix lib/test_bpf.c failures

[PATCH 0/5] s390/bpf: fix lib/test_bpf.c failures

[Ilya Leoshkevich]

This patch series fixes three regressions caused by 4e9b4a6883dd
("s390/bpf: Use relative long branches") reported by Seth Forshee
(patches 2-4) and adds two minor related improvements (patches 1
and 5).

Ilya Leoshkevich (5):
  selftests: bpf: test_kmod.sh: fix running out of srctree
  s390/bpf: fix sign extension in branch_ku
  s390/bpf: use brcl for jumping to exit_ip if necessary
  s390/bpf: tolerate not converging code shrinking
  s390/bpf: use bpf_skip() in bpf_jit_prologue()

 arch/s390/net/bpf_jit_comp.c             | 63 +++++++++++++++---------
 tools/testing/selftests/bpf/test_kmod.sh | 12 +++--
 2 files changed, 50 insertions(+), 25 deletions(-)

-- 
2.25.4

[PATCH 1/5] selftests: bpf: test_kmod.sh: fix running out of srctree

[Ilya Leoshkevich]

When running out of srctree, relative path to lib/test_bpf.ko is
different than when running in srctree. Check $building_out_of_srctree
environment variable and use a different relative path if needed.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 tools/testing/selftests/bpf/test_kmod.sh | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/bpf/test_kmod.sh b/tools/testing/selftests/bpf/test_kmod.sh
index 9df0d2ac45f8..4f6444bcd53f 100755
--- a/tools/testing/selftests/bpf/test_kmod.sh
+++ b/tools/testing/selftests/bpf/test_kmod.sh
@@ -10,7 +10,13 @@ if [ "$(id -u)" != "0" ]; then
 	exit $ksft_skip
 fi
 
-SRC_TREE=../../../../
+if [ "$building_out_of_srctree" ]; then
+	# We are in linux-build/kselftest/bpf
+	OUTPUT=../../
+else
+	# We are in linux/tools/testing/selftests/bpf
+	OUTPUT=../../../../
+fi
 
 test_run()
 {
@@ -19,8 +25,8 @@ test_run()
 
 	echo "[ JIT enabled:$1 hardened:$2 ]"
 	dmesg -C
-	if [ -f ${SRC_TREE}/lib/test_bpf.ko ]; then
-		insmod ${SRC_TREE}/lib/test_bpf.ko 2> /dev/null
+	if [ -f ${OUTPUT}/lib/test_bpf.ko ]; then
+		insmod ${OUTPUT}/lib/test_bpf.ko 2> /dev/null
 		if [ $? -ne 0 ]; then
 			rc=1
 		fi
-- 
2.25.4

[PATCH 2/5] s390/bpf: fix sign extension in branch_ku

[Ilya Leoshkevich]

Both signed and unsigned variants of BPF_JMP | BPF_K require
sign-extending the immediate. JIT emits cgfi for the signed case,
which is correct, and clgfi for the unsigned case, which is not
correct: clgfi zero-extends the immediate.

s390 does not provide an instruction that does sign-extension and
unsigned comparison at the same time. Therefore, fix by first loading
the sign-extended immediate into work register REG_1 and proceeding
as if it's BPF_X.

Fixes: 4e9b4a6883dd ("s390/bpf: Use relative long branches")
Reported-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 arch/s390/net/bpf_jit_comp.c | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index 8fe7bdfc8d15..67608f6092f8 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -1507,21 +1507,10 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		}
 		break;
 branch_ku:
-		is_jmp32 = BPF_CLASS(insn->code) == BPF_JMP32;
-		/* clfi or clgfi %dst,imm */
-		EMIT6_IMM(is_jmp32 ? 0xc20f0000 : 0xc20e0000,
-			  dst_reg, imm);
-		if (!is_first_pass(jit) &&
-		    can_use_rel(jit, addrs[i + off + 1])) {
-			/* brc mask,off */
-			EMIT4_PCREL_RIC(0xa7040000,
-					mask >> 12, addrs[i + off + 1]);
-		} else {
-			/* brcl mask,off */
-			EMIT6_PCREL_RILC(0xc0040000,
-					 mask >> 12, addrs[i + off + 1]);
-		}
-		break;
+		/* lgfi %w1,imm (load sign extend imm) */
+		src_reg = REG_1;
+		EMIT6_IMM(0xc0010000, src_reg, imm);
+		goto branch_xu;
 branch_xs:
 		is_jmp32 = BPF_CLASS(insn->code) == BPF_JMP32;
 		if (!is_first_pass(jit) &&
-- 
2.25.4

Re: [PATCH 2/5] s390/bpf: fix sign extension in branch_ku

[Seth Forshee]

On Fri, Jul 17, 2020 at 06:53:23PM +0200, Ilya Leoshkevich wrote:
> Both signed and unsigned variants of BPF_JMP | BPF_K require
> sign-extending the immediate. JIT emits cgfi for the signed case,
> which is correct, and clgfi for the unsigned case, which is not
> correct: clgfi zero-extends the immediate.
> 
> s390 does not provide an instruction that does sign-extension and
> unsigned comparison at the same time. Therefore, fix by first loading
> the sign-extended immediate into work register REG_1 and proceeding
> as if it's BPF_X.
> 
> Fixes: 4e9b4a6883dd ("s390/bpf: Use relative long branches")
> Reported-by: Seth Forshee <seth.forshee@canonical.com>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

This fixes the failing tests I was seeing. Thanks!

Tested-by: Seth Forshee <seth.forshee@canonical.com>

[PATCH 3/5] s390/bpf: use brcl for jumping to exit_ip if necessary

[Ilya Leoshkevich]

"BPF_MAXINSNS: Maximum possible literals" test causes panic with
bpf_jit_harden = 2. The reason is that BPF_JMP | BPF_EXIT is always
emitted as brc, however, after removal of JITed image size
limitations, brcl might be required.

Fix by using brcl when necessary.

Fixes: 4e9b4a6883dd ("s390/bpf: Use relative long branches")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 arch/s390/net/bpf_jit_comp.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index 67608f6092f8..625ebe32b2d1 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -1358,8 +1358,12 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		last = (i == fp->len - 1) ? 1 : 0;
 		if (last)
 			break;
-		/* j <exit> */
-		EMIT4_PCREL(0xa7f40000, jit->exit_ip - jit->prg);
+		if (!is_first_pass(jit) && can_use_rel(jit, jit->exit_ip))
+			/* brc 0xf, <exit> */
+			EMIT4_PCREL_RIC(0xa7040000, 0xf, jit->exit_ip);
+		else
+			/* brcl 0xf, <exit> */
+			EMIT6_PCREL_RILC(0xc0040000, 0xf, jit->exit_ip);
 		break;
 	/*
 	 * Branch relative (number of skipped instructions) to offset on
-- 
2.25.4

[PATCH 4/5] s390/bpf: tolerate not converging code shrinking

[Ilya Leoshkevich]

"BPF_MAXINSNS: Maximum possible literals" unnecessarily falls back to
the interpreter because of failing sanity check in bpf_set_addr. The
problem is that there are a lot of branches that can be shrunk, and
doing so opens up the possibility to shrink even more. This process
does not converge after 3 passes, causing code offsets to change during
the codegen pass, which must never happen.

Fix by inserting nops during codegen pass in order to preserve code
offets.

Fixes: 4e9b4a6883dd ("s390/bpf: Use relative long branches")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 arch/s390/net/bpf_jit_comp.c | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index 625ebe32b2d1..e68854ab27ae 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -490,6 +490,24 @@ static void save_restore_regs(struct bpf_jit *jit, int op, u32 stack_depth)
 	} while (re <= last);
 }
 
+static void bpf_skip(struct bpf_jit *jit, int size)
+{
+	if (size >= 6 && !is_valid_rel(size)) {
+		/* brcl 0xf,size */
+		EMIT6_PCREL_RIL(0xc0f4000000, size);
+		size -= 6;
+	} else if (size >= 4 && is_valid_rel(size)) {
+		/* brc 0xf,size */
+		EMIT4_PCREL(0xa7f40000, size);
+		size -= 4;
+	}
+	while (size >= 2) {
+		/* bcr 0,%0 */
+		_EMIT2(0x0700);
+		size -= 2;
+	}
+}
+
 /*
  * Emit function prologue
  *
@@ -1610,7 +1628,14 @@ static bool bpf_is_new_addr_sane(struct bpf_jit *jit, int i)
  */
 static int bpf_set_addr(struct bpf_jit *jit, int i)
 {
-	if (!bpf_is_new_addr_sane(jit, i))
+	int delta;
+
+	if (is_codegen_pass(jit)) {
+		delta = jit->prg - jit->addrs[i];
+		if (delta < 0)
+			bpf_skip(jit, -delta);
+	}
+	if (WARN_ON_ONCE(!bpf_is_new_addr_sane(jit, i)))
 		return -1;
 	jit->addrs[i] = jit->prg;
 	return 0;
-- 
2.25.4

[PATCH 5/5] s390/bpf: use bpf_skip() in bpf_jit_prologue()

[Ilya Leoshkevich]

Now that we have bpf_skip() for emitting nops, use it in
bpf_jit_prologue() in order to reduce code duplication.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 arch/s390/net/bpf_jit_comp.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index e68854ab27ae..be4b8532dd3c 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -520,10 +520,11 @@ static void bpf_jit_prologue(struct bpf_jit *jit, u32 stack_depth)
 		/* xc STK_OFF_TCCNT(4,%r15),STK_OFF_TCCNT(%r15) */
 		_EMIT6(0xd703f000 | STK_OFF_TCCNT, 0xf000 | STK_OFF_TCCNT);
 	} else {
-		/* j tail_call_start: NOP if no tail calls are used */
-		EMIT4_PCREL(0xa7f40000, 6);
-		/* bcr 0,%0 */
-		EMIT2(0x0700, 0, REG_0);
+		/*
+		 * There are no tail calls. Insert nops in order to have
+		 * tail_call_start at a predictable offset.
+		 */
+		bpf_skip(jit, 6);
 	}
 	/* Tail calls have to skip above initialization */
 	jit->tail_call_start = jit->prg;
-- 
2.25.4

Re: [PATCH 0/5] s390/bpf: fix lib/test_bpf.c failures

[Alexei Starovoitov]

On Fri, Jul 17, 2020 at 9:53 AM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
>
> This patch series fixes three regressions caused by 4e9b4a6883dd
> ("s390/bpf: Use relative long branches") reported by Seth Forshee
> (patches 2-4) and adds two minor related improvements (patches 1
> and 5).

Applied. Thanks