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[PATCH v3 2/4] asm-generic: Provide a fncpy() implementation

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  [PATCH v3 2/4] asm-generic: Provide a fncpy() implementation Florian Fainelli <f.fainelli@gmail.com> - 2017-06-17 02:10 +0200
    Re: [PATCH v3 2/4] asm-generic: Provide a fncpy() implementation Florian Fainelli <f.fainelli@gmail.com> - 2017-06-19 19:30 +0200
    Re: [PATCH v3 2/4] asm-generic: Provide a fncpy() implementation Russell King - ARM Linux <linux@armlinux.org.uk> - 2017-06-19 19:50 +0200
    Re: [PATCH v3 2/4] asm-generic: Provide a fncpy() implementation Florian Fainelli <florian.fainelli@broadcom.com> - 2017-06-19 23:00 +0200

#1668100 — [PATCH v3 2/4] asm-generic: Provide a fncpy() implementation

Define a generic fncpy() implementation largely based on the ARM version
that requires an 8 bytes alignment for the destination address where to
copy this function as well as the function's own address.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
---
 include/asm-generic/fncpy.h | 93 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 93 insertions(+)
 create mode 100644 include/asm-generic/fncpy.h

diff --git a/include/asm-generic/fncpy.h b/include/asm-generic/fncpy.h
new file mode 100644
index 000000000000..ec03b83b8535
--- /dev/null
+++ b/include/asm-generic/fncpy.h
@@ -0,0 +1,93 @@
+/*
+ * include/asm-generic/fncpy.h - helper macros for function body copying
+ *
+ * Copyright (C) 2011 Linaro Limited
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+/*
+ * These macros are intended for use when there is a need to copy a low-level
+ * function body into special memory.
+ *
+ * For example, when reconfiguring the SDRAM controller, the code doing the
+ * reconfiguration may need to run from SRAM.
+ *
+ * NOTE: that the copied function body must be entirely self-contained and
+ * position-independent in order for this to work properly.
+ *
+ * NOTE: in order for embedded literals and data to get referenced correctly,
+ * the alignment of functions must be preserved when copying.  To ensure this,
+ * the source and destination addresses for fncpy() must be aligned to a
+ * multiple of 8 bytes: you will be get a BUG() if this condition is not met.
+ * You will typically need a ".align 3" directive in the assembler where the
+ * function to be copied is defined, and ensure that your allocator for the
+ * destination buffer returns 8-byte-aligned pointers.
+ *
+ * Typical usage example:
+ *
+ * extern int f(args);
+ * extern uint32_t size_of_f;
+ * int (*copied_f)(args);
+ * void *sram_buffer;
+ *
+ * copied_f = fncpy(sram_buffer, &f, size_of_f);
+ *
+ * ... later, call the function: ...
+ *
+ * copied_f(args);
+ *
+ * The size of the function to be copied can't be determined from C:
+ * this must be determined by other means, such as adding assmbler directives
+ * in the file where f is defined.
+ */
+
+#ifndef __ASM_FNCPY_H
+#define __ASM_FNCPY_H
+
+#include <linux/types.h>
+#include <linux/string.h>
+
+#include <asm/bug.h>
+#include <asm/cacheflush.h>
+
+/*
+ * Minimum alignment requirement for the source and destination addresses
+ * for function copying.
+ */
+#define FNCPY_ALIGN 8
+
+#define fncpy(dest_buf, funcp, size) ({					\
+	uintptr_t __funcp_address;					\
+	typeof(funcp) __result;						\
+									\
+	asm("" : "=r" (__funcp_address) : "0" (funcp));			\
+									\
+	/*								\
+	 * Ensure alignment of source and destination addresses.	\
+	 */								\
+	BUG_ON((uintptr_t)(dest_buf) & (FNCPY_ALIGN - 1) ||		\
+		(__funcp_address & (FNCPY_ALIGN - 1)));			\
+									\
+	memcpy(dest_buf, (void const *)__funcp_address, size);		\
+	flush_icache_range((unsigned long)(dest_buf),			\
+		(unsigned long)(dest_buf) + (size));			\
+									\
+	asm("" : "=r" (__result)					\
+		: "0" ((uintptr_t)(dest_buf)));				\
+									\
+	__result;							\
+})
+
+#endif /* !__ASM_FNCPY_H */
-- 
2.9.3

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#1669532

On 06/19/2017 08:18 AM, Yury Norov wrote:
> On Mon, Jun 19, 2017 at 02:51:08AM +0300, Yury Norov wrote:
>> Hi Florian,
>>
>> Some questions and thoughts inline.
>>
>> Yury
>>
>> On Fri, Jun 16, 2017 at 05:07:42PM -0700, Florian Fainelli wrote:
>>> Define a generic fncpy() implementation largely based on the ARM version
>>> that requires an 8 bytes alignment for the destination address where to
>>> copy this function as well as the function's own address.
>>>
>>> Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
>>> ---
>>>  include/asm-generic/fncpy.h | 93 +++++++++++++++++++++++++++++++++++++++++++++
>>>  1 file changed, 93 insertions(+)
>>>  create mode 100644 include/asm-generic/fncpy.h
> 
> One else thing I forgot to ask - now you have the generic
> implementation for fncpy(), so do you really need to save arm
> version of it?

Yes, it needs to deal with the Thumb bit, and there is no point in
making the generic implementation extremely flexible to support that
special case.

Thanks
-- 
Florian

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#1669550

On Mon, Jun 19, 2017 at 06:18:18PM +0300, Yury Norov wrote:
> One else thing I forgot to ask - now you have the generic
> implementation for fncpy(), so do you really need to save arm
> version of it?

This was covered in the review of v1, which took the ARM version
and incorrectly used it as an asm-generic implementation.

I explicitly asked Florian _not_ to copy the ARM fncpy() version
to asm-generic because it has (surprise surprise) ARM specific
behaviours that do not belong in a cross-architecture generic
version.

Namely, the ARM specific behaviour that bit 0 of a code address is
used to signal whether the code should be executed as ARM code or
as Thumb code.

This behaviour has no meaning on other architectures (eg, x86)
where code addresses are not 32-bit aligned.

So, suggesting that the ARM fncpy() should be used as an asm-generic
version is completely absurd, and just because we have an asm-generic
version also does not mean ARM should use it.

Florian's approach to providing an asm-generic version, leaving the
ARM specific version is entirely correct and appropriate.

So, in answer to your question, yes, we need _both_ an ARM specific
version and an asm-generic version, where the ARM specific version is
different from the asm-generic version.  Purely because it needs
architecture specific details.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.

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#1669944

On 06/18/2017 04:51 PM, Yury Norov wrote:
> Hi Florian,
> 
> Some questions and thoughts inline.
> 
> Yury
> 
> On Fri, Jun 16, 2017 at 05:07:42PM -0700, Florian Fainelli wrote:
>> Define a generic fncpy() implementation largely based on the ARM version
>> that requires an 8 bytes alignment for the destination address where to
>> copy this function as well as the function's own address.
>>
>> Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
>> ---
>>  include/asm-generic/fncpy.h | 93 +++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 93 insertions(+)
>>  create mode 100644 include/asm-generic/fncpy.h
>>
>> diff --git a/include/asm-generic/fncpy.h b/include/asm-generic/fncpy.h
>> new file mode 100644
>> index 000000000000..ec03b83b8535
>> --- /dev/null
>> +++ b/include/asm-generic/fncpy.h
>> @@ -0,0 +1,93 @@
>> +/*
>> + * include/asm-generic/fncpy.h - helper macros for function body copying
>> + *
>> + * Copyright (C) 2011 Linaro Limited
>> + *
>> + * This program is free software; you can redistribute it and/or modify
>> + * it under the terms of the GNU General Public License version 2 as
>> + * published by the Free Software Foundation.
>> + *
>> + * This program is distributed in the hope that it will be useful,
>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> + * GNU General Public License for more details.
>> + *
>> + * You should have received a copy of the GNU General Public License
>> + * along with this program; if not, write to the Free Software
>> + * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
>> + */
>> +
>> +/*
>> + * These macros are intended for use when there is a need to copy a low-level
>> + * function body into special memory.
>> + *
>> + * For example, when reconfiguring the SDRAM controller, the code doing the
>> + * reconfiguration may need to run from SRAM.
>> + *
>> + * NOTE: that the copied function body must be entirely self-contained and
>> + * position-independent in order for this to work properly.
>> + *
>> + * NOTE: in order for embedded literals and data to get referenced correctly,
>> + * the alignment of functions must be preserved when copying.  To ensure this,
>> + * the source and destination addresses for fncpy() must be aligned to a
>> + * multiple of 8 bytes: you will be get a BUG() if this condition is not met.
>> + * You will typically need a ".align 3" directive in the assembler where the
>> + * function to be copied is defined, and ensure that your allocator for the
>> + * destination buffer returns 8-byte-aligned pointers.
>> + *
>> + * Typical usage example:
>> + *
>> + * extern int f(args);
>> + * extern uint32_t size_of_f;
>> + * int (*copied_f)(args);
>> + * void *sram_buffer;
>> + *
>> + * copied_f = fncpy(sram_buffer, &f, size_of_f);
>> + *
>> + * ... later, call the function: ...
>> + *
>> + * copied_f(args);
>> + *
>> + * The size of the function to be copied can't be determined from C:
>> + * this must be determined by other means, such as adding assmbler directives
>> + * in the file where f is defined.
>> + */
>> +
>> +#ifndef __ASM_FNCPY_H
>> +#define __ASM_FNCPY_H
>> +
>> +#include <linux/types.h>
>> +#include <linux/string.h>
>> +
>> +#include <asm/bug.h>
>> +#include <asm/cacheflush.h>
>> +
>> +/*
>> + * Minimum alignment requirement for the source and destination addresses
>> + * for function copying.
>> + */
>> +#define FNCPY_ALIGN 8
> 
> From now this is not arm-only, and it's possible that some architectures
> might want to redefine it in their arch/xxx/include/asm/fncpy.h files.
> So it will be easier for them if you'll wrap FNCPY_ALIGN here with #ifdef
> guards.
> 
> By the way, compiler already has an information on the proper alignment.
> Maybe it's better to use it as the default value here instead of the
> hardcoded value?
> 
> #ifndef FNCPY_ALIGN
> #define FNCPY_ALIGN ({void foo(); __alignof__(&foo);})
> #endif
> 
>> +
>> +#define fncpy(dest_buf, funcp, size) ({					\
> 
> Do you really need to check types inside the macro? If not, you can
> declare it as function, which is better in general, with the memcpy-like
> prototype.
> 
>> +	uintptr_t __funcp_address;					\
>> +	typeof(funcp) __result;						\
>> +									\
>> +	asm("" : "=r" (__funcp_address) : "0" (funcp));			\
>> +									\
>> +	/*								\
>> +	 * Ensure alignment of source and destination addresses.	\
>> +	 */								\
>> +	BUG_ON((uintptr_t)(dest_buf) & (FNCPY_ALIGN - 1) ||		\
> 
> People don't like new BUG_ONs. Maybe it's better to use BUILD_BUG_ON()
> at compile time and WARN_ON() at runtime?

If you have a BUILD_BUG_ON() what's the point of the WARN_ON()?

> 
>> +		(__funcp_address & (FNCPY_ALIGN - 1)));			\
> 
> There is IS_ALIGNED() macro for things like this.

Sure, makes sense.

> 
> And I frankly don't understand the 2nd check. One can imagine the
> situation when someone wants copy the function from the packed blob or
> some intermediate location were the function is unaligned, and it's
> impossible with the current implementation.

That's a good point, I am not sure if this is historical, or if there is
a reason for that from the ARM/Linux implementation. It sounds unlikely
that the function would be unaligned though considering that you'd have
to refer to the function being copied by its symbolic name, which
assumes it's in the kernel image or a module, and highly probable that
it is also aligned.

> 
>> +									\
>> +	memcpy(dest_buf, (void const *)__funcp_address, size);		\
>> +	flush_icache_range((unsigned long)(dest_buf),			\
>> +		(unsigned long)(dest_buf) + (size));			\
>> +									\
>> +	asm("" : "=r" (__result)					\
>> +		: "0" ((uintptr_t)(dest_buf)));				\
>> +									\
>> +	__result;							\
>> +})
>> +
>> +#endif /* !__ASM_FNCPY_H */
>> -- 
>> 2.9.3


-- 
Florian

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