[PATCH v4 1/4] x86/asm: move inline string functions to <asm/string_inline.h>

[Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>]

In a future patch, 'boot/string.c' will include inline string functions.

Using the header <asm/string.h> is problematic for a couple of reasons
(i.e., build errors), which can be addressed, but introduce unnecessary
complexity and regression risk (beyond these _found_ couple of reasons).

Using a new header <asm/string_inline.h> is simpler and transparent to
existing users of <asm/string.h>, with less changes to 'boot/string.c'
and its users (eg 'boot/compressed/string.c' and 'purgatory/purgatory.ro'),
which minimize regression risk.

No functional change intended.

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: 
https://lore.kernel.org/oe-kbuild-all/202605140922.q7IlUv7o-lkp@xxxxxxxxx/
Signed-off-by: Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>

---
(*) Reasons not to include <asm/string.h> in 'boot/string.c':

1) 'boot/string.c' is built for 16-bit/real mode thus some type and word
size errors happen when <asm/string.h> include, e.g., <asm/string_64.h>.

This can be addressed with '#ifndef _SETUP' (defined by 'boot/Makefile').

2) 'boot/string.c' is included by 'boot/compressed/string.c' which is
the source of 'purgatory/string.o', linked by 'purgatory/purgatory.ro'
(CONFIG_KEXEC_FILE).

In 64BIT, <asm/string.h> includes <asm/string_64.h>, which references
__memset() and __memmove() with KCFI_REFERENCE(), ie, __ADDRESSABLE();
however, 'purgatory/purgatory.ro' is not linked with implementations.

So, CONFIG_KEXEC_FILE and CONFIG_CFI without CONFIG_KASAN hit errors:

  >> ld.lld: error: undefined symbol: __memset
     >>> referenced by string.c
     >>>               arch/x86/purgatory/purgatory.ro:\
                       (__UNIQUE_ID_addressable___memset_0)
  --
  >> ld.lld: error: undefined symbol: __memmove
     >>> referenced by string.c
     >>>               arch/x86/purgatory/purgatory.ro:\
                       (__UNIQUE_ID_addressable___memmove_1)

(Note: this is not hit with CONFIG_KASAN because 'boot/compressed/string.c'
adds aliases __memset()/__memmove() to memset()/memmove() in that case.)

This can be addressed with 'CFLAGS_string.o := -D__DISABLE_EXPORTS' so to
disable KCFI_REFERENCE() in 'purgatory/Makefile' (it removes CC_FLAGS_CFI
anyway).

...

However, since a change in this series would need more changes to address
errors it causes, it is reasonable to change the series not to cause them,
by using a separate header with _just_ inline string functions.
---
 arch/x86/include/asm/string.h        | 21 +--------------------
 arch/x86/include/asm/string_inline.h | 26 ++++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 20 deletions(-)

diff --git a/arch/x86/include/asm/string.h b/arch/x86/include/asm/string.h
index 
9cb5aae7fba9ffcf0f5af8f939d30467750ccaa9..8a849bb5d0c7100a2a1820ec0af9fc9f850727fd
 100644
--- a/arch/x86/include/asm/string.h
+++ b/arch/x86/include/asm/string.h
@@ -8,25 +8,6 @@
 # include <asm/string_64.h>
 #endif
 
-static __always_inline void *__inline_memcpy(void *to, const void *from, 
size_t len)
-{
-       void *ret = to;
-
-       asm volatile("rep movsb"
-                    : "+D" (to), "+S" (from), "+c" (len)
-                    : : "memory");
-       return ret;
-}
-
-static __always_inline void *__inline_memset(void *s, int v, size_t n)
-{
-       void *ret = s;
-
-       asm volatile("rep stosb"
-                    : "+D" (s), "+c" (n)
-                    : "a" ((uint8_t)v)
-                    : "memory");
-       return ret;
-}
+#include <asm/string_inline.h>
 
 #endif /* _ASM_X86_STRING_H */
diff --git a/arch/x86/include/asm/string_inline.h 
b/arch/x86/include/asm/string_inline.h
new file mode 100644
index 
0000000000000000000000000000000000000000..ffcfa6583d2bc7b91d078888518d0e80248af940
--- /dev/null
+++ b/arch/x86/include/asm/string_inline.h
@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_STRING_INLINE_H
+#define _ASM_X86_STRING_INLINE_H
+
+static __always_inline void *__inline_memcpy(void *to, const void *from, 
size_t len)
+{
+       void *ret = to;
+
+       asm volatile("rep movsb"
+                    : "+D" (to), "+S" (from), "+c" (len)
+                    : : "memory");
+       return ret;
+}
+
+static __always_inline void *__inline_memset(void *s, int v, size_t n)
+{
+       void *ret = s;
+
+       asm volatile("rep stosb"
+                    : "+D" (s), "+c" (n)
+                    : "a" ((uint8_t)v)
+                    : "memory");
+       return ret;
+}
+
+#endif /* _ASM_X86_STRING_INLINE_H */

-- 
2.51.0