MIPS: prctl: Disallow FRE without FR with PR_SET_FP_MODE requests

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

Fixes: 9791554b45a2 ("MIPS,prctl: add PR_[GS]ET_FP_MODE prctl options for MIPS")
Signed-off-by: Maciej W. Rozycki <macro@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 4.0+
Patchwork: https://patchwork.linux-mips.org/patch/19327/
Signed-off-by: James Hogan <jhogan@kernel.org>

diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
index b9e9bf6288497b0b3a520860e6504cc9928e7824..3775a8d694fb0879ba0abed7b2edc34a06a52a28 100644 (file)
--- a/arch/mips/kernel/process.c
+++ b/arch/mips/kernel/process.c
@@ -721,6 +721,10 @@ int mips_set_process_fp_mode(struct task_struct *task, unsigned int value)
        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))