[tip: x86/cleanups] x86/elf: Use e_machine to select start_thread for x32

[tip-bot2 for Gabriel Krisman Bertazi]

The following commit has been merged into the x86/cleanups branch of tip:

Commit-ID:     2424b14605c71a7187c14edd525044eb36bdea47
Gitweb:        https://git.kernel.org/tip/2424b14605c71a7187c14edd525044eb36bdea47
Author:        Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
AuthorDate:    Sat, 03 Oct 2020 23:25:32 -04:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Mon, 26 Oct 2020 13:46:46 +01:00

x86/elf: Use e_machine to select start_thread for x32

Since TIF_X32 is going away, avoid using it to find the ELF type in
compat_start_thread.

According to SysV AMD64 ABI Draft, an AMD64 ELF object using ILP32 must
have ELFCLASS32 with (E_MACHINE == EM_X86_64), so use that ELF field to
differentiate a x32 object from a IA32 object when executing start_thread()
in compat mode.

Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20201004032536.1229030-7-krisman@xxxxxxxxxxxxx


---
 arch/x86/include/asm/elf.h   | 5 +++--
 arch/x86/kernel/process_64.c | 5 ++---
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 9220efc..109697a 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -186,8 +186,9 @@ static inline void elf_common_init(struct thread_struct *t,
 #define	COMPAT_ELF_PLAT_INIT(regs, load_addr)		\
 	elf_common_init(&current->thread, regs, __USER_DS)
 
-void compat_start_thread(struct pt_regs *regs, u32 new_ip, u32 new_sp);
-#define compat_start_thread compat_start_thread
+void compat_start_thread(struct pt_regs *regs, u32 new_ip, u32 new_sp, bool x32);
+#define COMPAT_START_THREAD(ex, regs, new_ip, new_sp)	\
+	compat_start_thread(regs, new_ip, new_sp, ex->e_machine == EM_X86_64)
 
 void set_personality_ia32(bool);
 #define COMPAT_SET_PERSONALITY(ex)			\
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index df342be..5fb4103 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -511,11 +511,10 @@ start_thread(struct pt_regs *regs, unsigned long new_ip, unsigned long new_sp)
 EXPORT_SYMBOL_GPL(start_thread);
 
 #ifdef CONFIG_COMPAT
-void compat_start_thread(struct pt_regs *regs, u32 new_ip, u32 new_sp)
+void compat_start_thread(struct pt_regs *regs, u32 new_ip, u32 new_sp, bool x32)
 {
 	start_thread_common(regs, new_ip, new_sp,
-			    test_thread_flag(TIF_X32)
-			    ? __USER_CS : __USER32_CS,
+			    x32 ? __USER_CS : __USER32_CS,
 			    __USER_DS, __USER_DS);
 }
 #endif