Re: [Qemu-devel] Fix for incorrect SYSRET instruction implementation -- anyone looked at this yet?

[Stefan Weil <sw@weilnetz.de>]

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Hi Bill,

sending text only e-mails might help. Usually git send-email is better 
than using KMail or other mail software.
Use tools like git gui to get a correct signature. Any optional personal 
comments should come directly after
the line with ---. Could you please re-send your patch? Check it before 
sending with scripts/checkpatch.pl.

Maybe this looks like much regulations to fix a bug, but some rules are 
necessary to keep a project
like QEMU alive.

Cc'ing Paolo and Richard because this might be an important bug fix for 
the next release.
(scripts/get_maintainer.pl tells which maintainers should be cc'ed).

Regards
Stefan


Am 09.03.2015 um 23:02 schrieb Bill Paul:
>
> Nobody has commented on this yet. According to my reading of the Intel 
> documentation, the SYSRET instruction is supposed to force the RPL 
> bits of the %ss register to 3 when returning to user mode. The actual 
> sequence is:
>
> SS.Selector <-- (IA32_STAR[63:48]+8) OR 3; (* RPL forced to 3 *)
>
> However, the code in helper_sysret() leaves them at 0 (in other words, 
> the "OR 3" part of the above sequence is missing). It does set the 
> privilege level bits of %cs correctly though.
>
> This has caused me trouble with some of my VxWorks development: code 
> that runs okay on real hardware will crash on QEMU, unless I apply the 
> patch below.
>
> Can someone confirm that this is in fact a real bug? The Intel 
> architecture manual seems quite clear about the SYSRET behavior. The 
> bug seems to have been around as far back as QEMU 0.10.5.
>
> I am using QEMU 2.2.0 on FreeBSD/amd64 9.1-RELEASE.
>
> -Bill
>
> Signed-off-by: Bill Paul <wpaul@windriver.com>
>
> ---
>
> target-i386/seg_helper.c | 4 ++--
>
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/target-i386/seg_helper.c b/target-i386/seg_helper.c
>
> index fa374d0..2bc757a 100644
>
> --- a/target-i386/seg_helper.c
>
> +++ b/target-i386/seg_helper.c
>
> @@ -1043,7 +1043,7 @@ void helper_sysret(CPUX86State *env, int dflag)
>
> DESC_CS_MASK | DESC_R_MASK | DESC_A_MASK);
>
> env->eip = (uint32_t)env->regs[R_ECX];
>
> }
>
> - cpu_x86_load_seg_cache(env, R_SS, selector + 8,
>
> + cpu_x86_load_seg_cache(env, R_SS, (selector + 8) | 3,
>
> 0, 0xffffffff,
>
> DESC_G_MASK | DESC_B_MASK | DESC_P_MASK |
>
> DESC_S_MASK | (3 << DESC_DPL_SHIFT) |
>
> @@ -1056,7 +1056,7 @@ void helper_sysret(CPUX86State *env, int dflag)
>
> DESC_S_MASK | (3 << DESC_DPL_SHIFT) |
>
> DESC_CS_MASK | DESC_R_MASK | DESC_A_MASK);
>
> env->eip = (uint32_t)env->regs[R_ECX];
>
> - cpu_x86_load_seg_cache(env, R_SS, selector + 8,
>
> + cpu_x86_load_seg_cache(env, R_SS, (selector + 8) | 3,
>
> 0, 0xffffffff,
>
> DESC_G_MASK | DESC_B_MASK | DESC_P_MASK |
>
> DESC_S_MASK | (3 << DESC_DPL_SHIFT) |
>
> -- 
>
> 1.8.0
>
> -- 
>
> =============================================================================
>
> -Bill Paul (510) 749-2329 | Senior Member of Technical Staff,
>
> wpaul@windriver.com | Master of Unix-Fu - Wind River Systems
>
> =============================================================================
>
> "I put a dollar in a change machine. Nothing changed." - George Carlin
>
> =============================================================================
>


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