Re: [PATCH] x86/vdso: Add prctl to set per-process VDSO load

On Tue, Sep 16, 2014 at 5:05 PM, Richard Larocque [off-list ref] wrote:

Adds new prctl calls to enable or disable VDSO loading for a process
and its children.

The PR_SET_DISABLE_VDSO call takes one argument, which is interpreted as
a boolean value.  If true, it disables the loading of the VDSO on exec()
for this process and any children created after this call.  A false
value unsets the flag.

The PR_GET_DISABLE_VDSO option returns a non-negative true value if VDSO
loading has been disabled for this process, zero if it has not been
disabled, and a negative value in case of error.

These prctl calls are hidden behind a new Kconfig,
CONFIG_VDSO_DISABLE_PRCTL.  This feature is available only on x86.

The command line option vdso=0 overrides the behavior of
PR_SET_DISABLE_VDSO, however, PR_GET_DISABLE_VDSO will coninue to return
whetever setting was last set with PR_SET_DISABLE_VDSO.

Signed-off-by: Richard Larocque <redacted>
---
This patch is part of some work to better handle times and CRIU migration.
I suspect that there are other use cases out there, so I'm offering this
patch separately.

When considering CRIU migration and times, we put some thought into how
to handle the rdtsc instruction.  If we migrate between machines or across
reboots, the migrated process will see values that could break its assumptions
about how rdtsc is supposed to work.

I don't get it.

If __vdso_clock_gettime returns the wrong value in any scenario, we
should fix that.  Simiarly, CRIU *already works*, unless there's
something I don't know of.

That being said, I would like an option to gate off RDTSC for a
process and its children in order to make PR_TSC_SIGSEGV more useful.
All the prerequisites are there now.

What problem are you trying to solve exactly?

--Andy