linux-kernel - Re: [PATCH] MIPS: Differentiate between 32 and 64 bit ELF header

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Message-ID: <56AF82AB.5010502@bmw-carit.de>
Date:	Mon, 1 Feb 2016 17:07:07 +0100
From:	Daniel Wagner <daniel.wagner@...-carit.de>
To:	"Maciej W. Rozycki" <macro@...tec.com>
CC:	<linux-mips@...ux-mips.org>, <linux-kernel@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [PATCH] MIPS: Differentiate between 32 and 64 bit ELF header

On 02/01/2016 01:52 AM, Maciej W. Rozycki wrote:
> On Fri, 29 Jan 2016, Daniel Wagner wrote:
> 
>> Depending on the configuration either the 32 or 64 bit version of
>> elf_check_arch() is defined. parse_crash_elf32_headers() does
>> some basic verification of the ELF header via elf_check_arch().
>> parse_crash_elf64_headers() does it via vmcore_elf64_check_arch()
>> which expands to the same elf_check_check().
>>
>>    In file included from include/linux/elf.h:4:0,
>>                     from fs/proc/vmcore.c:13:
>>    fs/proc/vmcore.c: In function 'parse_crash_elf64_headers':
>>>> arch/mips/include/asm/elf.h:228:23: error: initialization from incompatible pointer type [-Werror=incompatible-pointer-types]
>>      struct elfhdr *__h = (hdr);     \
>>                           ^
>>    include/linux/crash_dump.h:41:37: note: in expansion of macro 'elf_check_arch'
>>     #define vmcore_elf64_check_arch(x) (elf_check_arch(x) || vmcore_elf_check_arch_cross(x))
>>                                         ^
>>    fs/proc/vmcore.c:1015:4: note: in expansion of macro 'vmcore_elf64_check_arch'
>>       !vmcore_elf64_check_arch(&ehdr) ||
>>        ^
>>
>> Since the MIPS ELF header for 32 bit and 64 bit differ we need
>> to check accordingly.
> 
>  I fail to see how it can work as it stands given that `elf_check_arch' is 
> called from the same source file both on a pointer to `Elf32_Ehdr' and one 
> to `Elf64_Ehdr'.  However the MIPS implementations of `elf_check_arch' 
> only use an auxiliary variable to avoid multiple evaluation of a macro 
> argument and therefore instead I recommend the use of the usual approach
> taken in such a situation within a statement expression, that is to 
> declare the variable with `typeof' rather than an explicit type.  As an
> upside this will minimise code disruption as well.

Good point on the type for hdr. Thought elf_check_arch() implementation
differ on 32 bit and 64 bit implementation. I played a bit around and the
simplest version I found was this here:


diff --git a/arch/mips/include/asm/elf.h b/arch/mips/include/asm/elf.h
index b01a6ff..8c88238 100644
--- a/arch/mips/include/asm/elf.h
+++ b/arch/mips/include/asm/elf.h
@@ -205,8 +205,6 @@ struct mips_elf_abiflags_v0 {
 #define MIPS_ABI_FP_64		6	/* -mips32r2 -mfp64 */
 #define MIPS_ABI_FP_64A		7	/* -mips32r2 -mfp64 -mno-odd-spreg */
 
-#ifdef CONFIG_32BIT
-
 /*
  * In order to be sure that we don't attempt to execute an O32 binary which
  * requires 64 bit FP (FR=1) on a system which does not support it we refuse
@@ -225,23 +223,30 @@ struct mips_elf_abiflags_v0 {
 #define elf_check_arch(hdr)						\
 ({									\
 	int __res = 1;							\
-	struct elfhdr *__h = (hdr);					\
+	typeof(*(hdr)) *__h = (hdr);					\
 									\
 	if (__h->e_machine != EM_MIPS)					\
 		__res = 0;						\
-	if (__h->e_ident[EI_CLASS] != ELFCLASS32)			\
-		__res = 0;						\
-	if ((__h->e_flags & EF_MIPS_ABI2) != 0)				\
-		__res = 0;						\
-	if (((__h->e_flags & EF_MIPS_ABI) != 0) &&			\
-	    ((__h->e_flags & EF_MIPS_ABI) != EF_MIPS_ABI_O32))		\
-		__res = 0;						\
-	if (__h->e_flags & __MIPS_O32_FP64_MUST_BE_ZERO)		\
-		__res = 0;						\
+	if (__same_type(hdr, Elf32_Ehdr *)) {				\
+		if (__h->e_ident[EI_CLASS] != ELFCLASS32)		\
+			__res = 0;					\
+		if ((__h->e_flags & EF_MIPS_ABI2) != 0)			\
+			__res = 0;					\
+		if (((__h->e_flags & EF_MIPS_ABI) != 0) &&		\
+			((__h->e_flags & EF_MIPS_ABI) != EF_MIPS_ABI_O32)) \
+			__res = 0;					\
+		if (__h->e_flags & __MIPS_O32_FP64_MUST_BE_ZERO)	\
+			__res = 0;					\
+	} else if (__same_type(hdr, Elf64_Ehdr *)) {			\
+		if (__h->e_ident[EI_CLASS] != ELFCLASS64)		\
+			__res = 0;					\
+	}								\
 									\
 	__res;								\
 })
 
+#ifdef CONFIG_32BIT
+
 /*
  * These are used to set parameters in the core dumps.
  */
@@ -250,21 +255,6 @@ struct mips_elf_abiflags_v0 {
 #endif /* CONFIG_32BIT */
 
 #ifdef CONFIG_64BIT
-/*
- * This is used to ensure we don't load something for the wrong architecture.
- */
-#define elf_check_arch(hdr)						\
-({									\
-	int __res = 1;							\
-	struct elfhdr *__h = (hdr);					\
-									\
-	if (__h->e_machine != EM_MIPS)					\
-		__res = 0;						\
-	if (__h->e_ident[EI_CLASS] != ELFCLASS64)			\
-		__res = 0;						\
-									\
-	__res;								\
-})
 
 /*
  * These are used to set parameters in the core dumps.


Not sure if that is what you had in mind.

cheers,
daniel