[PATCH bpf v2 2/2] selftests/bpf: Check per-CPU address resolution per register

[Vineet Gupta <vineet.gupta@linux.dev>]

An ld_imm64 of a per-CPU map value is followed by a mov_percpu_addr that
reuses the same register, so which register the address lands in
decides how the JIT encodes the add. Getting the REX prefix wrong there
is invisible to a functional test on x86 unless the address happens to
land in an extended register, which is why this went unnoticed.

Add one __naked program per extended register, R5, R7, R8 and R9, each
loading a .percpu variable into that register, and match the JITed add
against the register it must resolve into. R1 is covered too, so that a
fix which sets REX.R unconditionally does not pass either.

Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
---
 .../selftests/bpf/prog_tests/verifier.c       |   2 +
 .../bpf/progs/verifier_percpu_addr.c          | 114 ++++++++++++++++++
 2 files changed, 116 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_percpu_addr.c

diff --git a/tools/testing/selftests/bpf/prog_tests/verifier.c b/tools/testing/selftests/bpf/prog_tests/verifier.c
index 8113fea7ba86..64ac49ad67e6 100644
--- a/tools/testing/selftests/bpf/prog_tests/verifier.c
+++ b/tools/testing/selftests/bpf/prog_tests/verifier.c
@@ -79,6 +79,7 @@
 #include "verifier_netfilter_retcode.skel.h"
 #include "verifier_bpf_fastcall.skel.h"
 #include "verifier_or_jmp32_k.skel.h"
+#include "verifier_percpu_addr.skel.h"
 #include "verifier_precision.skel.h"
 #include "verifier_prevent_map_lookup.skel.h"
 #include "verifier_private_stack.skel.h"
@@ -240,6 +241,7 @@ void test_verifier_netfilter_ctx(void)        { RUN(verifier_netfilter_ctx); }
 void test_verifier_netfilter_retcode(void)    { RUN(verifier_netfilter_retcode); }
 void test_verifier_bpf_fastcall(void)         { RUN(verifier_bpf_fastcall); }
 void test_verifier_or_jmp32_k(void)           { RUN(verifier_or_jmp32_k); }
+void test_verifier_percpu_addr(void)          { RUN(verifier_percpu_addr); }
 void test_verifier_precision(void)            { RUN(verifier_precision); }
 void test_verifier_prevent_map_lookup(void)   { RUN(verifier_prevent_map_lookup); }
 void test_verifier_private_stack(void)        { RUN(verifier_private_stack); }
diff --git a/tools/testing/selftests/bpf/progs/verifier_percpu_addr.c b/tools/testing/selftests/bpf/progs/verifier_percpu_addr.c
new file mode 100644
index 000000000000..962faea8ef90
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/verifier_percpu_addr.c
@@ -0,0 +1,114 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+int percpu_data SEC(".percpu");
+
+#if defined(__TARGET_ARCH_x86)
+
+/*
+ * An ld_imm64 of a per-CPU map value is followed by a mov_percpu_addr that
+ * reuses the same register, so the register the address lands in decides how
+ * the JIT encodes the add. On x86 R5, R7, R8 and R9 are the extended
+ * registers, whose high bit needs REX.R because the destination sits in
+ * ModRM.reg. Check one program per extended register, since getting the
+ * prefix wrong resolves the address into whichever register shares the low
+ * three bits instead.
+ */
+
+SEC("raw_tp")
+__description("per-CPU address into r5")
+__success
+__arch_x86_64
+__jited("	addq	%gs:{{.*}}, %r8")
+__naked void percpu_addr_into_r5(void)
+{
+	asm volatile ("					\
+	r5 = %[percpu_data] ll;				\
+	r0 = *(u32 *)(r5 + 0);				\
+	exit;						\
+"	:
+	: __imm_addr(percpu_data)
+	: __clobber_all);
+}
+
+SEC("raw_tp")
+__description("per-CPU address into r7")
+__success
+__arch_x86_64
+__jited("	addq	%gs:{{.*}}, %r13")
+__naked void percpu_addr_into_r7(void)
+{
+	asm volatile ("					\
+	r7 = %[percpu_data] ll;				\
+	r0 = *(u32 *)(r7 + 0);				\
+	exit;						\
+"	:
+	: __imm_addr(percpu_data)
+	: __clobber_all);
+}
+
+SEC("raw_tp")
+__description("per-CPU address into r8")
+__success
+__arch_x86_64
+__jited("	addq	%gs:{{.*}}, %r14")
+__naked void percpu_addr_into_r8(void)
+{
+	asm volatile ("					\
+	r8 = %[percpu_data] ll;				\
+	r0 = *(u32 *)(r8 + 0);				\
+	exit;						\
+"	:
+	: __imm_addr(percpu_data)
+	: __clobber_all);
+}
+
+SEC("raw_tp")
+__description("per-CPU address into r9")
+__success
+__arch_x86_64
+__jited("	addq	%gs:{{.*}}, %r15")
+__naked void percpu_addr_into_r9(void)
+{
+	asm volatile ("					\
+	r9 = %[percpu_data] ll;				\
+	r0 = *(u32 *)(r9 + 0);				\
+	exit;						\
+"	:
+	: __imm_addr(percpu_data)
+	: __clobber_all);
+}
+
+/* A register that needs no REX.R, to catch a fix that overcorrects. */
+SEC("raw_tp")
+__description("per-CPU address into r1")
+__success
+__arch_x86_64
+__jited("	addq	%gs:{{.*}}, %rdi")
+__naked void percpu_addr_into_r1(void)
+{
+	asm volatile ("					\
+	r1 = %[percpu_data] ll;				\
+	r0 = *(u32 *)(r1 + 0);				\
+	exit;						\
+"	:
+	: __imm_addr(percpu_data)
+	: __clobber_all);
+}
+
+#else
+
+SEC("raw_tp")
+__description("percpu addr dummy")
+__success
+int dummy_test(void)
+{
+	return 0;
+}
+
+#endif
+
+char _license[] SEC("license") = "GPL";
-- 
2.53.0-Meta