On 2015-08-29 17:49, Valdis Kletnieks wrote:
> Compiler warning:
>
> CC [M] arch/x86/kvm/emulate.o
> arch/x86/kvm/emulate.c: In function "__do_insn_fetch_bytes":
> arch/x86/kvm/emulate.c:814:9: warning: "linear" may be used uninitialized in this function [-Wmaybe-uninitialized]
>
> GCC is smart enough to realize that the inlined __linearize may return before
> setting the value of linear, but not smart enough to realize the same
> X86EMU_CONTINUE blocks actual use of the value. However, the value of
> 'linear' can only be set to one value, so hoisting the one line of code
> upwards makes GCC happy with the code.
>
> Reported-by: Aruna Hewapathirane <aruna.hewapathirane@xxxxxxxxx>
> Tested-by: Aruna Hewapathirane <aruna.hewapathirane@xxxxxxxxx>
> Signed-off-by: Valdis Kletnieks <valdis.kletnieks@xxxxxx>
>
> --- a/arch/x86/kvm/emulate.c.dist 2015-08-11 14:10:05.366061993 -0400
> +++ b/arch/x86/kvm/emulate.c 2015-08-29 13:43:13.014163958 -0400
> @@ -650,6 +650,7 @@ static __always_inline int __linearize(s
> u16 sel;
>
> la = seg_base(ctxt, addr.seg) + addr.ea;
> + *linear = la;
> *max_size = 0;
> switch (mode) {
> case X86EMUL_MODE_PROT64:
> @@ -693,7 +694,6 @@ static __always_inline int __linearize(s
> }
> if (insn_aligned(ctxt, size) && ((la & (size - 1)) != 0))
> return emulate_gp(ctxt, 0);
> - *linear = la;
> return X86EMUL_CONTINUE;
> bad:
> if (addr.seg == VCPU_SREG_SS)
>
Unfortunately this patch broke GNU/Hurd when running under KVM. It fails
to boot almost immediately. I haven't debug it more, but it looks like
*linear should not always be written. This can easily be reproduced by
trying to boot Debian Installer from this ISO:
http://ftp.debian-ports.org/debian-cd/hurd-i386/debian-hurd-2015/debian-hurd-2015-i386-CD-1.iso
Aurelien
--
Aurelien Jarno GPG: 4096R/1DDD8C9B
aurelien@xxxxxxxxxxx http://www.aurel32.net
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