Implement memcmp(), memcpy(), and memset() to override the compiler's
built-in versions in order to guarantee that the compiler won't generate
out-of-line calls to external functions via the PLT. This allows the
helpers to be safely used in guest code, as KVM selftests don't support
dynamic loading of guest code.
Steal the implementations from the kernel's generic versions, sans the
optimizations in memcmp() for unaligned accesses.
Put the utilities in a separate compilation unit and build with
-ffreestanding to fudge around a gcc "feature" where it will optimize
memset(), memcpy(), etc... by generating a recursive call. I.e. the
compiler optimizes itself into infinite recursion. Alternatively, the
individual functions could be tagged with
optimize("no-tree-loop-distribute-patterns"), but using "optimize" for
anything but debug is discouraged, and Linus NAK'd the use of the flag
in the kernel proper[*].
https://lore.kernel.org/lkml/CAHk-=wik-oXnUpfZ6Hw37uLykc-_P0Apyn2XuX-odh-3Nzop8w@mail.gmail.com
Cc: Andrew Jones <[email protected]>
Cc: Anup Patel <[email protected]>
Cc: Atish Patra <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Janosch Frank <[email protected]>
Cc: Claudio Imbrenda <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
---
tools/testing/selftests/kvm/Makefile | 11 +++++-
.../selftests/kvm/lib/string_override.c | 39 +++++++++++++++++++
2 files changed, 49 insertions(+), 1 deletion(-)
create mode 100644 tools/testing/selftests/kvm/lib/string_override.c
diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
index 8b1b32628ac8..681816df69cc 100644
--- a/tools/testing/selftests/kvm/Makefile
+++ b/tools/testing/selftests/kvm/Makefile
@@ -48,6 +48,8 @@ LIBKVM += lib/rbtree.c
LIBKVM += lib/sparsebit.c
LIBKVM += lib/test_util.c
+LIBKVM_STRING += lib/string_override.c
+
LIBKVM_x86_64 += lib/x86_64/apic.c
LIBKVM_x86_64 += lib/x86_64/handlers.S
LIBKVM_x86_64 += lib/x86_64/perf_test_util.c
@@ -221,7 +223,8 @@ LIBKVM_C := $(filter %.c,$(LIBKVM))
LIBKVM_S := $(filter %.S,$(LIBKVM))
LIBKVM_C_OBJ := $(patsubst %.c, $(OUTPUT)/%.o, $(LIBKVM_C))
LIBKVM_S_OBJ := $(patsubst %.S, $(OUTPUT)/%.o, $(LIBKVM_S))
-LIBKVM_OBJS = $(LIBKVM_C_OBJ) $(LIBKVM_S_OBJ)
+LIBKVM_STRING_OBJ := $(patsubst %.c, $(OUTPUT)/%.o, $(LIBKVM_STRING))
+LIBKVM_OBJS = $(LIBKVM_C_OBJ) $(LIBKVM_S_OBJ) $(LIBKVM_STRING_OBJ)
EXTRA_CLEAN += $(LIBKVM_OBJS) cscope.*
@@ -232,6 +235,12 @@ $(LIBKVM_C_OBJ): $(OUTPUT)/%.o: %.c
$(LIBKVM_S_OBJ): $(OUTPUT)/%.o: %.S
$(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c $< -o $@
+# Compile the string overrides as freestanding to prevent the compiler from
+# generating self-referential code, e.g. with "freestanding" the compiler may
+# "optimize" memcmp() by invoking memcmp(), thus causing infinite recursion.
+$(LIBKVM_STRING_OBJ): $(OUTPUT)/%.o: %.c
+ $(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c -ffreestanding $< -o $@
+
x := $(shell mkdir -p $(sort $(dir $(TEST_GEN_PROGS))))
$(TEST_GEN_PROGS): $(LIBKVM_OBJS)
$(TEST_GEN_PROGS_EXTENDED): $(LIBKVM_OBJS)
diff --git a/tools/testing/selftests/kvm/lib/string_override.c b/tools/testing/selftests/kvm/lib/string_override.c
new file mode 100644
index 000000000000..632398adc229
--- /dev/null
+++ b/tools/testing/selftests/kvm/lib/string_override.c
@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <stddef.h>
+
+/*
+ * Override the "basic" built-in string helpers so that they can be used in
+ * guest code. KVM selftests don't support dynamic loading in guest code and
+ * will jump into the weeds if the compiler decides to insert an out-of-line
+ * call via the PLT.
+ */
+int memcmp(const void *cs, const void *ct, size_t count)
+{
+ const unsigned char *su1, *su2;
+ int res = 0;
+
+ for (su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--) {
+ if ((res = *su1 - *su2) != 0)
+ break;
+ }
+ return res;
+}
+
+void *memcpy(void *dest, const void *src, size_t count)
+{
+ char *tmp = dest;
+ const char *s = src;
+
+ while (count--)
+ *tmp++ = *s++;
+ return dest;
+}
+
+void *memset(void *s, int c, size_t count)
+{
+ char *xs = s;
+
+ while (count--)
+ *xs++ = c;
+ return s;
+}
--
2.37.3.998.g577e59143f-goog
On Wed, Sep 28, 2022 at 11:36:46PM +0000, Sean Christopherson wrote:
> Implement memcmp(), memcpy(), and memset() to override the compiler's
> built-in versions in order to guarantee that the compiler won't generate
> out-of-line calls to external functions via the PLT. This allows the
> helpers to be safely used in guest code, as KVM selftests don't support
> dynamic loading of guest code.
>
> Steal the implementations from the kernel's generic versions, sans the
> optimizations in memcmp() for unaligned accesses.
>
> Put the utilities in a separate compilation unit and build with
> -ffreestanding to fudge around a gcc "feature" where it will optimize
> memset(), memcpy(), etc... by generating a recursive call. I.e. the
> compiler optimizes itself into infinite recursion. Alternatively, the
> individual functions could be tagged with
> optimize("no-tree-loop-distribute-patterns"), but using "optimize" for
> anything but debug is discouraged, and Linus NAK'd the use of the flag
> in the kernel proper[*].
>
> https://lore.kernel.org/lkml/CAHk-=wik-oXnUpfZ6Hw37uLykc-_P0Apyn2XuX-odh-3Nzop8w@mail.gmail.com
>
> Cc: Andrew Jones <[email protected]>
> Cc: Anup Patel <[email protected]>
> Cc: Atish Patra <[email protected]>
> Cc: Christian Borntraeger <[email protected]>
> Cc: Janosch Frank <[email protected]>
> Cc: Claudio Imbrenda <[email protected]>
> Signed-off-by: Sean Christopherson <[email protected]>
> ---
> tools/testing/selftests/kvm/Makefile | 11 +++++-
> .../selftests/kvm/lib/string_override.c | 39 +++++++++++++++++++
> 2 files changed, 49 insertions(+), 1 deletion(-)
> create mode 100644 tools/testing/selftests/kvm/lib/string_override.c
>
> diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
> index 8b1b32628ac8..681816df69cc 100644
> --- a/tools/testing/selftests/kvm/Makefile
> +++ b/tools/testing/selftests/kvm/Makefile
> @@ -48,6 +48,8 @@ LIBKVM += lib/rbtree.c
> LIBKVM += lib/sparsebit.c
> LIBKVM += lib/test_util.c
>
> +LIBKVM_STRING += lib/string_override.c
> +
> LIBKVM_x86_64 += lib/x86_64/apic.c
> LIBKVM_x86_64 += lib/x86_64/handlers.S
> LIBKVM_x86_64 += lib/x86_64/perf_test_util.c
> @@ -221,7 +223,8 @@ LIBKVM_C := $(filter %.c,$(LIBKVM))
> LIBKVM_S := $(filter %.S,$(LIBKVM))
> LIBKVM_C_OBJ := $(patsubst %.c, $(OUTPUT)/%.o, $(LIBKVM_C))
> LIBKVM_S_OBJ := $(patsubst %.S, $(OUTPUT)/%.o, $(LIBKVM_S))
> -LIBKVM_OBJS = $(LIBKVM_C_OBJ) $(LIBKVM_S_OBJ)
> +LIBKVM_STRING_OBJ := $(patsubst %.c, $(OUTPUT)/%.o, $(LIBKVM_STRING))
> +LIBKVM_OBJS = $(LIBKVM_C_OBJ) $(LIBKVM_S_OBJ) $(LIBKVM_STRING_OBJ)
>
> EXTRA_CLEAN += $(LIBKVM_OBJS) cscope.*
>
> @@ -232,6 +235,12 @@ $(LIBKVM_C_OBJ): $(OUTPUT)/%.o: %.c
> $(LIBKVM_S_OBJ): $(OUTPUT)/%.o: %.S
> $(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c $< -o $@
>
> +# Compile the string overrides as freestanding to prevent the compiler from
> +# generating self-referential code, e.g. with "freestanding" the compiler may
^ without
> +# "optimize" memcmp() by invoking memcmp(), thus causing infinite recursion.
> +$(LIBKVM_STRING_OBJ): $(OUTPUT)/%.o: %.c
> + $(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c -ffreestanding $< -o $@
> +
> x := $(shell mkdir -p $(sort $(dir $(TEST_GEN_PROGS))))
> $(TEST_GEN_PROGS): $(LIBKVM_OBJS)
> $(TEST_GEN_PROGS_EXTENDED): $(LIBKVM_OBJS)
> diff --git a/tools/testing/selftests/kvm/lib/string_override.c b/tools/testing/selftests/kvm/lib/string_override.c
> new file mode 100644
> index 000000000000..632398adc229
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/lib/string_override.c
> @@ -0,0 +1,39 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +#include <stddef.h>
> +
> +/*
> + * Override the "basic" built-in string helpers so that they can be used in
> + * guest code. KVM selftests don't support dynamic loading in guest code and
> + * will jump into the weeds if the compiler decides to insert an out-of-line
> + * call via the PLT.
> + */
> +int memcmp(const void *cs, const void *ct, size_t count)
> +{
> + const unsigned char *su1, *su2;
> + int res = 0;
> +
> + for (su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--) {
> + if ((res = *su1 - *su2) != 0)
> + break;
> + }
> + return res;
> +}
> +
> +void *memcpy(void *dest, const void *src, size_t count)
> +{
> + char *tmp = dest;
> + const char *s = src;
> +
> + while (count--)
> + *tmp++ = *s++;
> + return dest;
> +}
> +
> +void *memset(void *s, int c, size_t count)
> +{
> + char *xs = s;
> +
> + while (count--)
> + *xs++ = c;
> + return s;
> +}
> --
> 2.37.3.998.g577e59143f-goog
>
Otherwise
Reviewed-by: Andrew Jones <[email protected]>
On Thu, Sep 29, 2022, Andrew Jones wrote:
> On Wed, Sep 28, 2022 at 11:36:46PM +0000, Sean Christopherson wrote:
> > @@ -232,6 +235,12 @@ $(LIBKVM_C_OBJ): $(OUTPUT)/%.o: %.c
> > $(LIBKVM_S_OBJ): $(OUTPUT)/%.o: %.S
> > $(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c $< -o $@
> >
> > +# Compile the string overrides as freestanding to prevent the compiler from
> > +# generating self-referential code, e.g. with "freestanding" the compiler may
> ^ without
Oof, good eyes. Thanks!