Re: [RFC PATCH v2 6/6] prctl.2: Add tagged address ABI control prctls (arm64)

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On Tue, Jun 09, 2020 at 01:04:25PM +0200, Michael Kerrisk (man-pages) wrote:
> Do we have any review comments for this (extensive!) patch from Dave?

(Adding Andrey, since he was involved with this ABI)

Regardless, it would be good to have Catalin's ack and I think he was
planning to take a look at this.

Will

> On 5/27/20 11:17 PM, Dave Martin wrote:
> > ** This patch is a draft for review and should not be applied before it
> >    has been discussed. **
> > 
> > Add documentation for the the PR_SET_TAGGED_ADDR_CTRL and
> > PR_GET_TAGGED_ADDR_CTRL prctls added in Linux 5.4 for arm64.
> > 
> > Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Cc: Will Deacon <will@xxxxxxxxxx>
> > Cc: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> > ---
> > 
> >  man2/prctl.2 | 156 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 156 insertions(+)
> > 
> > diff --git a/man2/prctl.2 b/man2/prctl.2
> > index 3ee2702..062fd51 100644
> > --- a/man2/prctl.2
> > +++ b/man2/prctl.2
> > @@ -1504,6 +1504,143 @@ For more information, see the kernel source file
> >  (or
> >  .I Documentation/arm64/sve.txt
> >  before Linux 5.3).
> > +.\" prctl PR_SET_TAGGED_ADDR_CTRL
> > +.\" commit 63f0c60379650d82250f22e4cf4137ef3dc4f43d
> > +.TP
> > +.BR PR_SET_TAGGED_ADDR_CTRL " (since Linux 5.4, only on arm64)"
> > +Controls support for passing tagged userspace addresses to the kernel
> > +(i.e., addresses where bits 56\(em63 are not all zero).
> > +.IP
> > +The level of support is selected by
> > +.IR "(unsigned int) arg2" ,
> > +which can be one of the following:
> > +.RS
> > +.TP
> > +.B 0
> > +Addresses that are passed
> > +for the purpose of being dereferenced by the kernel
> > +must be untagged.
> > +.TP
> > +.B PR_TAGGED_ADDR_ENABLE
> > +Addresses that are passed
> > +for the purpose of being dereferenced by the kernel
> > +may be tagged, with the exceptions summarized below.
> > +.RE
> > +.IP
> > +The remaining arguments
> > +.IR arg3 ", " arg4 " and " arg5
> > +must all be zero.
> > +.IP
> > +On success, the mode specified in
> > +.I arg2
> > +is set for the calling thread and the the return value is 0.
> > +If the arguments are invalid,
> > +the mode specified in
> > +.I arg2
> > +is unrecognized,
> > +or if this feature is disabled or unsupported by the kernel,
> > +the call fails with
> > +.BR EINVAL .
> > +.IP
> > +In particular, if
> > +.BR prctl ( PR_SET_TAGGED_ADDR_CTRL ,
> > +0, 0, 0, 0)
> > +fails with
> > +.B EINVAL
> > +then all addresses passed to the kernel must be untagged.
> > +.IP
> > +Irrespective of which mode is set,
> > +addresses passed to certain interfaces
> > +must always be untagged:
> > +.RS
> > +.IP \(em
> > +.BR brk (2),
> > +.BR mmap (2),
> > +.BR shmat (2),
> > +and the
> > +.I new_address
> > +argument of
> > +.BR mremap (2).
> > +.IP
> > +(Prior to Linux 5.6 these accepted tagged addresses,
> > +but the behaviour may not be what you expect.
> > +Don't rely on it.)
> > +.IP \(em
> > +\(oqpolymorphic\(cq interfaces
> > +that accept pointers to arbitrary types cast to a
> > +.I void *
> > +or other generic type, specifically
> > +.BR prctl (2),
> > +.BR ioctl (2),
> > +and in general
> > +.BR setsockopt (2)
> > +(only certain specific
> > +.BR setsockopt (2)
> > +options allow tagged addresses).
> > +.IP \(em
> > +.BR shmdt (2).
> > +.RE
> > +.IP
> > +This list of exclusions may shrink
> > +when moving from one kernel version to a later kernel version.
> > +While the kernel may make some guarantees
> > +for backwards compatibility reasons,
> > +for the purposes of new software
> > +the effect of passing tagged addresses to these interfaces
> > +is unspecified.
> > +.IP
> > +The mode set by this call is inherited across
> > +.BR fork (2)
> > +and
> > +.BR clone (2).
> > +The mode is reset by
> > +.BR execve (2)
> > +to 0
> > +(i.e., tagged addresses not permitted in the user/kernel ABI).
> > +.IP
> > +.B Warning:
> > +Because the compiler or run-time environment
> > +may make use of address tagging,
> > +a successful
> > +.B PR_SET_TAGGED_ADDR_CTRL
> > +may crash the calling process.
> > +The conditions for using it safely are complex and system-dependent.
> > +Don't use it unless you know what you are doing.
> > +.IP
> > +For more information, see the kernel source file
> > +.IR Documentation/arm64/tagged\-address\-abi.rst .
> > +.\" prctl PR_GET_TAGGED_ADDR_CTRL
> > +.\" commit 63f0c60379650d82250f22e4cf4137ef3dc4f43d
> > +.TP
> > +.BR PR_GET_TAGGED_ADDR_CTRL " (since Linux 5.4, only on arm64)"
> > +Returns the current tagged address mode
> > +for the calling thread.
> > +.IP
> > +Arguments
> > +.IR arg2 ", " arg3 ", " arg4 " and " arg5
> > +must all be zero.
> > +.IP
> > +If the arguments are invalid
> > +or this feature is disabled or unsupported by the kernel,
> > +the call fails with
> > +.BR EINVAL .
> > +In particular, if
> > +.BR prctl ( PR_GET_TAGGED_ADDR_CTRL ,
> > +0, 0, 0, 0)
> > +fails with
> > +.BR EINVAL ,
> > +then this feature is definitely unsupported or disabled,
> > +and all addresses passed to the kernel must be untagged.
> > +.IP
> > +Otherwise, the call returns a nonnegative value
> > +describing the current tagged address mode,
> > +encoded in the same way as the
> > +.I arg2
> > +argument of
> > +.BR PR_SET_TAGGED_ADDR_CTRL .
> > +.IP
> > +For more information, see the kernel source file
> > +.IR Documentation/arm64/tagged\-address\-abi.rst .
> >  .\"
> >  .\" prctl PR_TASK_PERF_EVENTS_DISABLE
> >  .TP
> > @@ -1749,6 +1886,7 @@ On success,
> >  .BR PR_GET_SPECULATION_CTRL ,
> >  .BR PR_SVE_GET_VL ,
> >  .BR PR_SVE_SET_VL ,
> > +.BR PR_GET_TAGGED_ADDR_CTRL ,
> >  .BR PR_GET_THP_DISABLE ,
> >  .BR PR_GET_TIMING ,
> >  .BR PR_GET_TIMERSLACK ,
> > @@ -2057,6 +2195,24 @@ is
> >  .B PR_SVE_GET_VL
> >  and SVE is not available on this platform.
> >  .TP
> > +.B EINVAL
> > +.I option
> > +is
> > +.BR PR_SET_TAGGED_ADDR_CTRL
> > +and the arguments are invalid or unsupported.
> > +See the description of
> > +.B PR_SET_TAGGED_ADDR_CTRL
> > +above for details.
> > +.TP
> > +.B EINVAL
> > +.I option
> > +is
> > +.BR PR_GET_TAGGED_ADDR_CTRL
> > +and the arguments are invalid or unsupported.
> > +See the description of
> > +.B PR_GET_TAGGED_ADDR_CTRL
> > +above for details.
> > +.TP
> >  .B ENODEV
> >  .I option
> >  was
> > 
> 
> 
> -- 
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/



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