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

[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 <address@hidden>

---

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

--

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-Bill Paul (510) 749-2329 | Senior Member of Technical Staff,

address@hidden | Master of Unix-Fu - Wind River Systems

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"I put a dollar in a change machine. Nothing changed." - George Carlin

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