Re: [PATCH v3 2/2] prctl.2: Add tagged address ABI control prctls (arm64)

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hi Dave,

On 6/24/20 7:36 PM, Dave Martin wrote:
> 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@arm.com>
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>

Thanks for another nicely written patch! I've applied locally, 
but won't push just yet, to allow for some (more) reviews/acks
to come in.

Thanks,

Michael

> ---
> 
> Kept Catalin's Reviewed-by, since the changes are pretty minor.
> 
> Changes since v2:
> 
>  * Clarified type of PR_SET_TAGGED_ADDR_CTRL arg2.
> 
>  * Added Linux commit where enforecement of zeroing reserved args was
>    added for PR_SET_TAGGED_ADDR_CTRL.
> 
>  * Added explicit text regarding how to disable the tagged-address ABI
>    globally through sysctl.
> 
>  * Rearrange the tagged argument syscall exclusion list so that shmdt()
>    isn't called out specially.  The reader probably doesn't care about
>    this history here.
> 
>  * Minor rewordings.
> ---
>  man2/prctl.2 | 161 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 161 insertions(+)
> 
> diff --git a/man2/prctl.2 b/man2/prctl.2
> index 46ea9d2..cf92f3b 100644
> --- a/man2/prctl.2
> +++ b/man2/prctl.2
> @@ -1522,6 +1522,148 @@ 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 "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.
> +.\" Enforcement added in
> +.\" commit 3e91ec89f527b9870fe42dcbdb74fd389d123a95
> +.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 unsupported by the kernel
> +or disabled via
> +.IR /proc/sys/abi/tagged_addr_disabled ,
> +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),
> +.BR shmdt (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).
> +.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
> +For more information, see the kernel source file
> +.IR Documentation/arm64/tagged\-address\-abi.rst .
> +.IP
> +.B Warning:
> +This call is primarily intended for use by the run-time environment.
> +A successful
> +.B PR_SET_TAGGED_ADDR_CTRL
> +call elsewhere 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.
> +.\" 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 either unsupported,
> +or disabled via
> +.IR /proc/sys/abi/tagged_addr_disabled .
> +In this case,
> +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
> @@ -1767,6 +1909,7 @@ 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 ,
> @@ -2074,6 +2217,24 @@ 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/