path: root/Documentation
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authorDave Hansen <dave.hansen@linux.intel.com>2014-12-12 10:38:35 -0800
committerThomas Gleixner <tglx@linutronix.de>2014-12-15 15:58:57 +0100
commit010e593b140decdc16bb0c1c754b07b1fccb6999 (patch)
tree960306184273c7ffd4480548b7631fa95e940b5d /Documentation
parentx86_64/traps: Fix always true condition (diff)
x86, mpx: Update documentation
I was writing some MPX test programs and realized that the current design makes it tricky. I did something like: bndcfgu |= bnd_dir | BNDCFGU_ENABLE; xrstor(); printf("xrstor done"); // #BR bounds exception here prctl(MPX_ENABLE_MANAGEMENT); and then compiled the app with "-fcheck-pointer-bounds -mmpx" to enable MPX instrumentation. The problem is that there is MPX instrumentation inserted in to the area of the printf(). The kernel gets a bounds exception and since management isn't yet enabled, it SIGSEGV's. Add a bit to the documentation to explain a way around this and where apps need to be careful. Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Cc: Dave Hansen <dave@sr71.net> Link: http://lkml.kernel.org/r/20141212183835.8C581B3E@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Diffstat (limited to 'Documentation')
1 files changed, 9 insertions, 3 deletions
diff --git a/Documentation/x86/intel_mpx.txt b/Documentation/x86/intel_mpx.txt
index 4472ed2ad921..6ca6e2bd9ae9 100644
--- a/Documentation/x86/intel_mpx.txt
+++ b/Documentation/x86/intel_mpx.txt
@@ -30,9 +30,15 @@ is how we expect the compiler, application and kernel to work together.
instrumentation as well as some setup code called early after the app
starts. New instruction prefixes are noops for old CPUs.
2) That setup code allocates (virtual) space for the "bounds directory",
- points the "bndcfgu" register to the directory and notifies the kernel
- (via the new prctl(PR_MPX_ENABLE_MANAGEMENT)) that the app will be using
- MPX.
+ points the "bndcfgu" register to the directory (must also set the valid
+ bit) and notifies the kernel (via the new prctl(PR_MPX_ENABLE_MANAGEMENT))
+ that the app will be using MPX. The app must be careful not to access
+ the bounds tables between the time when it populates "bndcfgu" and
+ when it calls the prctl(). This might be hard to guarantee if the app
+ is compiled with MPX. You can add "__attribute__((bnd_legacy))" to
+ the function to disable MPX instrumentation to help guarantee this.
+ Also be careful not to call out to any other code which might be
+ MPX-instrumented.
3) The kernel detects that the CPU has MPX, allows the new prctl() to
succeed, and notes the location of the bounds directory. Userspace is
expected to keep the bounds directory at that locationWe note it