On 12/21/20 9:06 PM, Gabriel Krisman Bertazi wrote:
> Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
>
> ---
> Changes since v2:
> (suggested by Alejandro Colomar)
> - selective -> selectively
> - Add missing oxford comma.
>
> Changes since v1:
> (suggested by Alejandro Colomar)
> - Use semantic lines
> - Fix usage of .{B|I}R and .{B|I}
> - Don't format literals
> - Fix preferred spelling of userspace
> - Fix case of word
> ---
> man2/prctl.2 | 100 +++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 100 insertions(+)
>
> diff --git a/man2/prctl.2 b/man2/prctl.2
> index f25f05fdb593..8105c2a0c9de 100644
> --- a/man2/prctl.2
> +++ b/man2/prctl.2
> @@ -1533,6 +1533,78 @@ For more information, see the kernel source file
> (or
> .I Documentation/arm64/sve.txt
> before Linux 5.3).
> +.TP
> +.\" prctl PR_SET_SYSCALL_USER_DISPATCH
> +.\" commit 1446e1df9eb183fdf81c3f0715402f1d7595d4
> +.BR PR_SET_SYSCALL_USER_DISPATCH " (since Linux 5.11, x86 only)"
> +.IP
> +Configure the Syscall User Dispatch mechanism
> +for the calling thread,
> +to selectively intercept system calls
> +and dispatch them back to user space through
> +.IR SIGSYS .
s/IR/BR/
I think a bit more explanation would help here. Could you add a
few sentences to explain what "dispatch back to user space through
SIGSYS" means?
> +.IP
> +The current Syscall User Dispatch mode is selected via
> +.IR arg2 ,
> +which can either be set to
> +.B PR_SYS_DISPATCH_ON
> +to enable the feature,
> +or to
> +.B PR_SYS_DISPATCH_OFF
> +to turn it off.
By the way, why is there no PR_*G*ET_SYSCALL_USER_DISPATCH?
> +.IP
> +With
> +.I arg2
> +set to
> +.BR PR_SYS_DISPATCH_ON ,
> +.I arg3
> +and
> +.I arg4
> +respectively identify the
> +.I offset
> +and
> +.I length
> +of a memory region in the process map
> +from where system calls are always allowed to be executed,
> +regardless of the switch variable.
Can there be more than one of these regions?
The text should be explicit about this, I think.
> +.I arg5
> +points to a char-sized variable
> +that is a fast switch to enable/disable the mechanism
> +without invoking the kernel.
> +The variable pointed by
> +.I arg5
> +can either be set to
> +.B PR_SYS_DISPATCH_ON
> +to enable the mechanism
> +or to
> +.B PR_SYS_DISPATCH_OFF
> +to temporarily disable it.
> +Any other value will fail the application
> +with a
> +.IR SIGSYS .
How/when does that failure occur? At the time of the
prctl() call, I presume? What if the caller later sets
the variable pointed to by arg5 to another value?
> +.IP
> +When a system call is intercepted,
> +.I SIGSYS
> +is raised with
> +.I si_code
> +set to
> +.BR SYS_USER_DISPATCH .
> +.IP
> +When
> +.I arg2
> +is set to
> +.BR PR_SYS_DISPATCH_OFF ,
> +the remaining arguments must be set to 0.
> +.IP
> +The setting is not preserved across
> +.BR fork (2),
> +.BR clone (2),
> +or
> +.BR execve (2).
> +.IP
> +For more information,
> +see the kernel source file
> +.IR Documentation/admin-guide/syscall-user-dispatch.rst
> .\" prctl PR_SET_TAGGED_ADDR_CTRL
> .\" commit 63f0c60379650d82250f22e4cf4137ef3dc4f43d
> .TP
> @@ -2000,6 +2072,14 @@ and
> .I arg3
> is an invalid address.
> .TP
> +.B EFAULT
> +.I option
> +is
> +.B PR_SET_SYSCALL_USER_DISPATCH
> +and
> +.I arg5
> +has an invalid address.
> +.TP
> .B EINVAL
> The value of
> .I option
> @@ -2229,6 +2309,26 @@ is
> and SVE is not available on this platform.
> .TP
> .B EINVAL
> +.I option is
> +.B PR_SET_SYSCALL_USER_DISPATCH
> +and one of the following is true:
> +.RS
> +.IP * 3
> +.I arg2
> +is
> +.B PR_SYS_DISPATCH_OFF
> +and remaining arguments are not 0;
s/and/and the/
> +.IP * 3
> +.I arg2
> +is
> +.B PR_SYS_DISPATCH_ON
> +and the memory region provided is invalid;
Please define "invalid"
> +.IP * 3
> +.I arg2
> +is invalid.
> +.RE
> +.TP
> +.B EINVAL
> .I option
> is
> .BR PR_SET_TAGGED_ADDR_CTRL
Thanks,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
