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31ac34ca56
So sparse rightfully complains that the u64 MSR value we're writing into the STAR MSR, i.e. 0xc0000081, is being truncated: ./arch/x86/include/asm/msr.h:193:36: warning: cast truncates bits from constant value (23001000000000 becomes 0) because the actual value doesn't fit into the unsigned 32-bit quantity which are the @low and @high wrmsrl() parameters. This is not a problem, practically, because gcc is actually being smart enough here and does the right thing: .loc 3 87 0 xorl %esi, %esi # we needz a 32-bit zero movl $2293776, %edx # 0x00230010 == (__USER32_CS << 16) | __KERNEL_CS go into the high bits movl $-1073741695, %ecx # MSR_STAR, i.e., 0xc0000081 movl %esi, %eax # low order 32 bits in the MSR which are 0 #APP # 87 "./arch/x86/include/asm/msr.h" 1 wrmsr More specifically, MSR_STAR[31:0] is being set to 0. That field is reserved on Intel and on AMD it is 32-bit SYSCALL Target EIP. I'd strongly guess because Intel doesn't have SYSCALL in compat/legacy mode and we're using SYSENTER and INT80 there. And for compat syscalls in long mode we use CSTAR. So let's fix the sparse warning by writing SYSRET and SYSCALL CS and SS into the high 32-bit half of STAR and 0 in the low half explicitly. [ Actually, if we had to be precise, we would have to read what's in STAR[31:0] and write it back unchanged on Intel and write 0 on AMD. I guess the current writing to 0 is still ok since Intel can apparently stomach it. ] The resulting code is identical to what we have above: .loc 3 87 0 xorl %esi, %esi # tmp104 movl $2293776, %eax #, tmp103 movl $-1073741695, %ecx #, tmp102 movl %esi, %edx # tmp104, tmp104 ... wrmsr Signed-off-by: Borislav Petkov <bp@suse.de> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Andy Lutomirski <luto@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Brian Gerst <brgerst@gmail.com> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/1448273546-2567-6-git-send-email-bp@alien8.de Signed-off-by: Ingo Molnar <mingo@kernel.org> |
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