[PATCH] x86/boot/compressed/64: fix warning for 32-bit trampoline copy

[PATCH] x86/boot/compressed/64: fix warning for 32-bit trampoline copy

[Arnd Bergmann]

gcc-8 warns that we copy TRAMPOLINE_32BIT_CODE_SIZE bytes from the pointer
to the function into actual trampoline, when that pointer is only 8 bytes:

In file included from arch/x86/boot/compressed/pgtable_64.c:3:
arch/x86/boot/compressed/pgtable_64.c: In function 'paging_prepare':
arch/x86/boot/compressed/../string.h:18:23: error: '__builtin_memcpy' reading 96 bytes from a region of size 8 [-Werror=stringop-overflow=]
 #define memcpy(d,s,l) __builtin_memcpy(d,s,l)
                       ^~~~~~~~~~~~~~~~~~~~~~~
arch/x86/boot/compressed/pgtable_64.c:62:2: note: in expansion of macro 'memcpy'
  memcpy(trampoline + TRAMPOLINE_32BIT_CODE_OFFSET / sizeof(unsigned long),
  ^~~~~~

I assume what was intended here is to copy the trampoline itself rather than
the pointer to the trampoline.

Cc: Martin Sebor <msebor@gmail.com>
Fixes: b91993a87aff ("x86/boot/compressed/64: Prepare trampoline memory")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
This is a fairly new warning in gcc, it's possible that the code is
correct and the warning message got it wrong, please double-check this.

Was the 32-bit trampoline code in the 64-bit decompressor tested successfully?
---
 arch/x86/boot/compressed/pgtable_64.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
index dad5da7b4c1a..35a0542fe397 100644
--- a/arch/x86/boot/compressed/pgtable_64.c
+++ b/arch/x86/boot/compressed/pgtable_64.c
@@ -60,7 +60,7 @@ struct paging_config paging_prepare(void)
 
 	/* Copy trampoline code in place */
 	memcpy(trampoline + TRAMPOLINE_32BIT_CODE_OFFSET / sizeof(unsigned long),
-			&trampoline_32bit_src, TRAMPOLINE_32BIT_CODE_SIZE);
+			trampoline_32bit_src, TRAMPOLINE_32BIT_CODE_SIZE);
 
 	/*
 	 * Set up a new page table that will be used for switching from 4-
-- 
2.9.0

Re: [PATCH] x86/boot/compressed/64: fix warning for 32-bit trampoline copy

[Kirill A. Shutemov]

On Thu, Feb 22, 2018 at 11:40:41AM +0100, Arnd Bergmann wrote:
> gcc-8 warns that we copy TRAMPOLINE_32BIT_CODE_SIZE bytes from the pointer
> to the function into actual trampoline, when that pointer is only 8 bytes:
> 
> In file included from arch/x86/boot/compressed/pgtable_64.c:3:
> arch/x86/boot/compressed/pgtable_64.c: In function 'paging_prepare':
> arch/x86/boot/compressed/../string.h:18:23: error: '__builtin_memcpy' reading 96 bytes from a region of size 8 [-Werror=stringop-overflow=]
>  #define memcpy(d,s,l) __builtin_memcpy(d,s,l)
>                        ^~~~~~~~~~~~~~~~~~~~~~~
> arch/x86/boot/compressed/pgtable_64.c:62:2: note: in expansion of macro 'memcpy'
>   memcpy(trampoline + TRAMPOLINE_32BIT_CODE_OFFSET / sizeof(unsigned long),
>   ^~~~~~
> 
> I assume what was intended here is to copy the trampoline itself rather than
> the pointer to the trampoline.
> 
> Cc: Martin Sebor <msebor@gmail.com>
> Fixes: b91993a87aff ("x86/boot/compressed/64: Prepare trampoline memory")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> This is a fairly new warning in gcc, it's possible that the code is
> correct and the warning message got it wrong, please double-check this.
> 
> Was the 32-bit trampoline code in the 64-bit decompressor tested successfully?

Yes, it was. Actually it tested on every boot of any 64-bit machine.

The fix is wrong and it breaks boot. The bug is in trampoline_32bit_src
declaration. Could you check if this helps with the warning:

diff --git a/arch/x86/boot/compressed/pgtable.h b/arch/x86/boot/compressed/pgtable.h
index 6e0db2260147..5e0b1c4abef4 100644
--- a/arch/x86/boot/compressed/pgtable.h
+++ b/arch/x86/boot/compressed/pgtable.h
@@ -12,7 +12,7 @@

 #ifndef __ASSEMBLER__

-extern void (*trampoline_32bit_src)(void *return_ptr);
+extern void trampoline_32bit_src(void *return_ptr);

 #endif /* __ASSEMBLER__ */
 #endif /* BOOT_COMPRESSED_PAGETABLE_H */
-- 
 Kirill A. Shutemov

Re: [PATCH] x86/boot/compressed/64: fix warning for 32-bit trampoline copy

[Arnd Bergmann]

On Thu, Feb 22, 2018 at 12:02 PM, Kirill A. Shutemov
<kirill@shutemov.name> wrote:
> On Thu, Feb 22, 2018 at 11:40:41AM +0100, Arnd Bergmann wrote:
>> gcc-8 warns that we copy TRAMPOLINE_32BIT_CODE_SIZE bytes from the pointer
>> to the function into actual trampoline, when that pointer is only 8 bytes:
>>
>> In file included from arch/x86/boot/compressed/pgtable_64.c:3:
>> arch/x86/boot/compressed/pgtable_64.c: In function 'paging_prepare':
>> arch/x86/boot/compressed/../string.h:18:23: error: '__builtin_memcpy' reading 96 bytes from a region of size 8 [-Werror=stringop-overflow=]
>>  #define memcpy(d,s,l) __builtin_memcpy(d,s,l)
>>                        ^~~~~~~~~~~~~~~~~~~~~~~
>> arch/x86/boot/compressed/pgtable_64.c:62:2: note: in expansion of macro 'memcpy'
>>   memcpy(trampoline + TRAMPOLINE_32BIT_CODE_OFFSET / sizeof(unsigned long),
>>   ^~~~~~
>>
>> I assume what was intended here is to copy the trampoline itself rather than
>> the pointer to the trampoline.
>>
>> Cc: Martin Sebor <msebor@gmail.com>
>> Fixes: b91993a87aff ("x86/boot/compressed/64: Prepare trampoline memory")
>> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>> ---
>> This is a fairly new warning in gcc, it's possible that the code is
>> correct and the warning message got it wrong, please double-check this.
>>
>> Was the 32-bit trampoline code in the 64-bit decompressor tested successfully?
>
> Yes, it was. Actually it tested on every boot of any 64-bit machine.
>
> The fix is wrong and it breaks boot. The bug is in trampoline_32bit_src
> declaration. Could you check if this helps with the warning:

Got it, yes that definitely explains the problem. Your patch fixes it
and I'll follow up with a new submission using this change, unless you
want to send it as a patch yourself.

       Arnd

> diff --git a/arch/x86/boot/compressed/pgtable.h b/arch/x86/boot/compressed/pgtable.h
> index 6e0db2260147..5e0b1c4abef4 100644
> --- a/arch/x86/boot/compressed/pgtable.h
> +++ b/arch/x86/boot/compressed/pgtable.h
> @@ -12,7 +12,7 @@
>
>  #ifndef __ASSEMBLER__
>
> -extern void (*trampoline_32bit_src)(void *return_ptr);
> +extern void trampoline_32bit_src(void *return_ptr);
>
>  #endif /* __ASSEMBLER__ */
>  #endif /* BOOT_COMPRESSED_PAGETABLE_H */
> --
>  Kirill A. Shutemov