Having PR_FP_MODE_FRE (i.e. Config5.FRE) set without PR_FP_MODE_FR (i.e. Status.FR) is not supported as the lone purpose of Config5.FRE is to emulate Status.FR=0 handling on FPU hardware that has Status.FR=1 hardwired[1][2]. Also we do not handle this case elsewhere, and assume throughout our code that TIF_HYBRID_FPREGS and TIF_32BIT_FPREGS cannot be set both at once for a task, leading to inconsistent behaviour if this does happen.
Return unsuccessfully then from prctl(2) PR_SET_FP_MODE calls requesting PR_FP_MODE_FRE to be set with PR_FP_MODE_FR clear. This corresponds to modes allowed by `mips_set_personality_fp'.
References:
[1] "MIPS Architecture For Programmers, Vol. III: MIPS32 / microMIPS32 Privileged Resource Architecture", Imagination Technologies, Document Number: MD00090, Revision 6.02, July 10, 2015, Table 9.69 "Config5 Register Field Descriptions", p. 262
[2] "MIPS Architecture For Programmers, Volume III: MIPS64 / microMIPS64 Privileged Resource Architecture", Imagination Technologies, Document Number: MD00091, Revision 6.03, December 22, 2015, Table 9.72 "Config5 Register Field Descriptions", p. 288
Cc: stable@vger.kernel.org # 4.0+ Fixes: 9791554b45a2 ("MIPS,prctl: add PR_[GS]ET_FP_MODE prctl options for MIPS") Signed-off-by: Maciej W. Rozycki macro@mips.com --- arch/mips/kernel/process.c | 4 ++++ 1 file changed, 4 insertions(+)
linux-mips-set-process-fp-mode-fr-fre.diff Index: linux/arch/mips/kernel/process.c =================================================================== --- linux.orig/arch/mips/kernel/process.c 2018-05-12 22:52:11.000000000 +0100 +++ linux/arch/mips/kernel/process.c 2018-05-12 23:07:15.147112000 +0100 @@ -721,6 +721,10 @@ int mips_set_process_fp_mode(struct task if (value & ~known_bits) return -EOPNOTSUPP;
+ /* Setting FRE without FR is not supported. */ + if ((value & (PR_FP_MODE_FR | PR_FP_MODE_FRE)) == PR_FP_MODE_FRE) + return -EOPNOTSUPP; + /* Avoid inadvertently triggering emulation */ if ((value & PR_FP_MODE_FR) && raw_cpu_has_fpu && !(raw_current_cpu_data.fpu_id & MIPS_FPIR_F64))
Hi Maciej,
On Tue, May 15, 2018 at 11:04:44PM +0100, Maciej W. Rozycki wrote:
Having PR_FP_MODE_FRE (i.e. Config5.FRE) set without PR_FP_MODE_FR (i.e. Status.FR) is not supported as the lone purpose of Config5.FRE is to emulate Status.FR=0 handling on FPU hardware that has Status.FR=1 hardwired[1][2]. Also we do not handle this case elsewhere, and assume throughout our code that TIF_HYBRID_FPREGS and TIF_32BIT_FPREGS cannot be set both at once for a task, leading to inconsistent behaviour if this does happen.
Reviewing the code I think we should actually end up with FR=1 in this case, because neither __own_fpu() nor the FPU emulator depend on the value of TIF_32BIT_FPREGS if TIF_HYBRID_FPREGS is set. So it's not too awful & I don't see the kernel doing anything too crazy, but it definitely isn't what the user asked for.
Return unsuccessfully then from prctl(2) PR_SET_FP_MODE calls requesting PR_FP_MODE_FRE to be set with PR_FP_MODE_FR clear. This corresponds to modes allowed by `mips_set_personality_fp'.
Looks good to me:
Reviewed-by: Paul Burton paul.burton@mips.com
Thanks, Paul
References:
[1] "MIPS Architecture For Programmers, Vol. III: MIPS32 / microMIPS32 Privileged Resource Architecture", Imagination Technologies, Document Number: MD00090, Revision 6.02, July 10, 2015, Table 9.69 "Config5 Register Field Descriptions", p. 262
[2] "MIPS Architecture For Programmers, Volume III: MIPS64 / microMIPS64 Privileged Resource Architecture", Imagination Technologies, Document Number: MD00091, Revision 6.03, December 22, 2015, Table 9.72 "Config5 Register Field Descriptions", p. 288
Cc: stable@vger.kernel.org # 4.0+ Fixes: 9791554b45a2 ("MIPS,prctl: add PR_[GS]ET_FP_MODE prctl options for MIPS") Signed-off-by: Maciej W. Rozycki macro@mips.com
arch/mips/kernel/process.c | 4 ++++ 1 file changed, 4 insertions(+)
linux-mips-set-process-fp-mode-fr-fre.diff Index: linux/arch/mips/kernel/process.c =================================================================== --- linux.orig/arch/mips/kernel/process.c 2018-05-12 22:52:11.000000000 +0100 +++ linux/arch/mips/kernel/process.c 2018-05-12 23:07:15.147112000 +0100 @@ -721,6 +721,10 @@ int mips_set_process_fp_mode(struct task if (value & ~known_bits) return -EOPNOTSUPP;
- /* Setting FRE without FR is not supported. */
- if ((value & (PR_FP_MODE_FR | PR_FP_MODE_FRE)) == PR_FP_MODE_FRE)
return -EOPNOTSUPP;- /* Avoid inadvertently triggering emulation */ if ((value & PR_FP_MODE_FR) && raw_cpu_has_fpu && !(raw_current_cpu_data.fpu_id & MIPS_FPIR_F64))
Hi Paul,
Having PR_FP_MODE_FRE (i.e. Config5.FRE) set without PR_FP_MODE_FR (i.e. Status.FR) is not supported as the lone purpose of Config5.FRE is to emulate Status.FR=0 handling on FPU hardware that has Status.FR=1 hardwired[1][2]. Also we do not handle this case elsewhere, and assume throughout our code that TIF_HYBRID_FPREGS and TIF_32BIT_FPREGS cannot be set both at once for a task, leading to inconsistent behaviour if this does happen.
Reviewing the code I think we should actually end up with FR=1 in this case, because neither __own_fpu() nor the FPU emulator depend on the value of TIF_32BIT_FPREGS if TIF_HYBRID_FPREGS is set. So it's not too awful & I don't see the kernel doing anything too crazy, but it definitely isn't what the user asked for.
However `arch_ptrace' does check TIF_32BIT_FPREGS and gets things wrong if TIF_HYBRID_FPREGS is also set. Which is actually how I discovered this issue.
Return unsuccessfully then from prctl(2) PR_SET_FP_MODE calls requesting PR_FP_MODE_FRE to be set with PR_FP_MODE_FR clear. This corresponds to modes allowed by `mips_set_personality_fp'.
Looks good to me:
Reviewed-by: Paul Burton paul.burton@mips.com
Thanks for your review.
Maciej
On Tue, May 15, 2018 at 11:04:44PM +0100, Maciej W. Rozycki wrote:
Having PR_FP_MODE_FRE (i.e. Config5.FRE) set without PR_FP_MODE_FR (i.e. Status.FR) is not supported as the lone purpose of Config5.FRE is to emulate Status.FR=0 handling on FPU hardware that has Status.FR=1 hardwired[1][2]. Also we do not handle this case elsewhere, and assume throughout our code that TIF_HYBRID_FPREGS and TIF_32BIT_FPREGS cannot be set both at once for a task, leading to inconsistent behaviour if this does happen.
Return unsuccessfully then from prctl(2) PR_SET_FP_MODE calls requesting PR_FP_MODE_FRE to be set with PR_FP_MODE_FR clear. This corresponds to modes allowed by `mips_set_personality_fp'.
References:
[1] "MIPS Architecture For Programmers, Vol. III: MIPS32 / microMIPS32 Privileged Resource Architecture", Imagination Technologies, Document Number: MD00090, Revision 6.02, July 10, 2015, Table 9.69 "Config5 Register Field Descriptions", p. 262
[2] "MIPS Architecture For Programmers, Volume III: MIPS64 / microMIPS64 Privileged Resource Architecture", Imagination Technologies, Document Number: MD00091, Revision 6.03, December 22, 2015, Table 9.72 "Config5 Register Field Descriptions", p. 288
Cc: stable@vger.kernel.org # 4.0+ Fixes: 9791554b45a2 ("MIPS,prctl: add PR_[GS]ET_FP_MODE prctl options for MIPS") Signed-off-by: Maciej W. Rozycki macro@mips.com
Thanks, applied to mips-fixes, hopefully for 4.17 (but if it misses tomorrows linux-next it may have to wait 'til 4.18).
Cheers James
linux-stable-mirror@lists.linaro.org
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James Hogan -
Maciej W. Rozycki -
Paul Burton