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([2001:a61:2467:2f01:faca:3d43:5e40:30d1]) by smtp.gmail.com with ESMTPSA id e17sm31581896wrw.84.2020.12.22.11.17.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 22 Dec 2020 11:17:41 -0800 (PST) Cc: mtk.manpages@gmail.com, linux-man@vger.kernel.org Subject: Re: [PATCH v3] prctl.2: Document Syscall User Dispatch To: Gabriel Krisman Bertazi , alx.manpages@gmail.com References: <20201221200622.3507690-1-krisman@collabora.com> From: "Michael Kerrisk (man-pages)" Message-ID: <9a2fa87e-2fc3-a8ad-0be6-6ddcfcdea8fa@gmail.com> Date: Tue, 22 Dec 2020 20:17:40 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: <20201221200622.3507690-1-krisman@collabora.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org On 12/21/20 9:06 PM, Gabriel Krisman Bertazi wrote: > Signed-off-by: Gabriel Krisman Bertazi > > --- > 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/