KVM: selftests: Compare insn opcodes directly in fix_hypercall_test

Directly compare the expected versus observed hypercall instructions when
verifying that KVM patched in the native hypercall (FIX_HYPERCALL_INSN
quirk enabled).  gcc rightly complains that doing a 4-byte memcpy() with
an "unsigned char" as the source generates an out-of-bounds accesses.

Alternatively, "exp" and "obs" could be declared as 3-byte arrays, but
there's no known reason to copy locally instead of comparing directly.

In function ‘assert_hypercall_insn’,
    inlined from ‘guest_main’ at x86_64/fix_hypercall_test.c:91:2:
x86_64/fix_hypercall_test.c:63:9: error: array subscript ‘unsigned int[0]’
 is partly outside array bounds of ‘unsigned char[1]’ [-Werror=array-bounds]
   63 |         memcpy(&exp, exp_insn, sizeof(exp));
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
x86_64/fix_hypercall_test.c: In function ‘guest_main’:
x86_64/fix_hypercall_test.c:42:22: note: object ‘vmx_hypercall_insn’ of size 1
   42 | extern unsigned char vmx_hypercall_insn;
      |                      ^~~~~~~~~~~~~~~~~~
x86_64/fix_hypercall_test.c:25:22: note: object ‘svm_hypercall_insn’ of size 1
   25 | extern unsigned char svm_hypercall_insn;
      |                      ^~~~~~~~~~~~~~~~~~
In function ‘assert_hypercall_insn’,
    inlined from ‘guest_main’ at x86_64/fix_hypercall_test.c:91:2:
x86_64/fix_hypercall_test.c:64:9: error: array subscript ‘unsigned int[0]’
 is partly outside array bounds of ‘unsigned char[1]’ [-Werror=array-bounds]
   64 |         memcpy(&obs, obs_insn, sizeof(obs));
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
x86_64/fix_hypercall_test.c: In function ‘guest_main’:
x86_64/fix_hypercall_test.c:25:22: note: object ‘svm_hypercall_insn’ of size 1
   25 | extern unsigned char svm_hypercall_insn;
      |                      ^~~~~~~~~~~~~~~~~~
x86_64/fix_hypercall_test.c:42:22: note: object ‘vmx_hypercall_insn’ of size 1
   42 | extern unsigned char vmx_hypercall_insn;
      |                      ^~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make: *** [../lib.mk:135: tools/testing/selftests/kvm/x86_64/fix_hypercall_test] Error 1

Fixes: 6c2fa8b20d0c ("selftests: KVM: Test KVM_X86_QUIRK_FIX_HYPERCALL_INSN")
Cc: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Message-Id: <20220928233652.783504-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c b/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c
index b1905d2..e0004bd 100644 (file)
--- a/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c
+++ b/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c
@@ -14,6 +14,9 @@
 #include "kvm_util.h"
 #include "processor.h"
 
+/* VMCALL and VMMCALL are both 3-byte opcodes. */
+#define HYPERCALL_INSN_SIZE    3
+
 static bool ud_expected;
 
 static void guest_ud_handler(struct ex_regs *regs)
@@ -22,7 +25,7 @@ static void guest_ud_handler(struct ex_regs *regs)
        GUEST_DONE();
 }
 
-extern unsigned char svm_hypercall_insn;
+extern uint8_t svm_hypercall_insn[HYPERCALL_INSN_SIZE];
 static uint64_t svm_do_sched_yield(uint8_t apic_id)
 {
        uint64_t ret;
@@ -39,7 +42,7 @@ static uint64_t svm_do_sched_yield(uint8_t apic_id)
        return ret;
 }
 
-extern unsigned char vmx_hypercall_insn;
+extern uint8_t vmx_hypercall_insn[HYPERCALL_INSN_SIZE];
 static uint64_t vmx_do_sched_yield(uint8_t apic_id)
 {
        uint64_t ret;
@@ -56,30 +59,20 @@ static uint64_t vmx_do_sched_yield(uint8_t apic_id)
        return ret;
 }
 
-static void assert_hypercall_insn(unsigned char *exp_insn, unsigned char *obs_insn)
-{
-       uint32_t exp = 0, obs = 0;
-
-       memcpy(&exp, exp_insn, sizeof(exp));
-       memcpy(&obs, obs_insn, sizeof(obs));
-
-       GUEST_ASSERT_EQ(exp, obs);
-}
-
 static void guest_main(void)
 {
-       unsigned char *native_hypercall_insn, *hypercall_insn;
+       uint8_t *native_hypercall_insn, *hypercall_insn;
        uint8_t apic_id;
 
        apic_id = GET_APIC_ID_FIELD(xapic_read_reg(APIC_ID));
 
        if (is_intel_cpu()) {
-               native_hypercall_insn = &vmx_hypercall_insn;
-               hypercall_insn = &svm_hypercall_insn;
+               native_hypercall_insn = vmx_hypercall_insn;
+               hypercall_insn = svm_hypercall_insn;
                svm_do_sched_yield(apic_id);
        } else if (is_amd_cpu()) {
-               native_hypercall_insn = &svm_hypercall_insn;
-               hypercall_insn = &vmx_hypercall_insn;
+               native_hypercall_insn = svm_hypercall_insn;
+               hypercall_insn = vmx_hypercall_insn;
                vmx_do_sched_yield(apic_id);
        } else {
                GUEST_ASSERT(0);
@@ -87,8 +80,13 @@ static void guest_main(void)
                return;
        }
 
+       /*
+        * The hypercall didn't #UD (guest_ud_handler() signals "done" if a #UD
+        * occurs).  Verify that a #UD is NOT expected and that KVM patched in
+        * the native hypercall.
+        */
        GUEST_ASSERT(!ud_expected);
-       assert_hypercall_insn(native_hypercall_insn, hypercall_insn);
+       GUEST_ASSERT(!memcmp(native_hypercall_insn, hypercall_insn, HYPERCALL_INSN_SIZE));
        GUEST_DONE();
 }