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From:   Gabriel Krisman Bertazi <krisman@collabora.com>
To:     luto@kernel.org, tglx@linutronix.de
Cc:     hpa@zytor.com, bp@alien8.de, rric@kernel.org, peterz@infradead.org,
        mingo@redhat.com, x86@kernel.org, linux-kernel@vger.kernel.org,
        Gabriel Krisman Bertazi <krisman@collabora.com>,
        kernel@collabora.com
Subject: [PATCH 5/6] x86: elf: Use e_machine to select start_thread for x32
Date:   Sat, 12 Sep 2020 03:05:52 -0400
Message-Id: <20200912070553.330622-6-krisman@collabora.com>
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Since TIF_X32 is going away, avoid using it to find the ELF type on
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@collabora.com>
---
 arch/x86/include/asm/elf.h   | 11 +++++++++--
 arch/x86/kernel/process_64.c | 11 +++++++----
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 9220efc65d78..33c1c9be2e07 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -186,8 +186,15 @@ 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_ia32(struct pt_regs *regs, u32 new_ip, u32 new_sp);
+void compat_start_thread_x32(struct pt_regs *regs, u32 new_ip, u32 new_sp);
+#define compat_start_thread(regs, new_ip, new_sp)			\
+do {									\
+	if (elf_ex->e_machine == EM_X86_64)				\
+		compat_start_thread_x32(regs, new_ip, new_sp);		\
+	else								\
+		compat_start_thread_ia32(regs, new_ip, new_sp);		\
+} while (0)
 
 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 9afefe325acb..56e882c339e6 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -511,12 +511,15 @@ 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_ia32(struct pt_regs *regs, u32 new_ip, u32 new_sp)
 {
 	start_thread_common(regs, new_ip, new_sp,
-			    test_thread_flag(TIF_X32)
-			    ? __USER_CS : __USER32_CS,
-			    __USER_DS, __USER_DS);
+			    __USER32_CS, __USER_DS, __USER_DS);
+}
+void compat_start_thread_x32(struct pt_regs *regs, u32 new_ip, u32 new_sp)
+{
+	start_thread_common(regs, new_ip, new_sp,
+			    __USER_CS, __USER_DS, __USER_DS);
 }
 #endif
 
-- 
2.28.0