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([2001:a61:251f:d701:c8c9:6ecf:205c:abb7]) by smtp.gmail.com with ESMTPSA id g7sm7365690wrq.21.2020.02.08.03.58.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 08 Feb 2020 03:58:08 -0800 (PST) Cc: mtk.manpages@gmail.com, Andrew Morton , Andy Lutomirski , dancol@google.com, Jann Horn , John Stultz , kernel-team@android.com, linux-api@vger.kernel.org, linux-man@vger.kernel.org, linux-mm@kvack.org, Matthew Wilcox , Mike Kravetz , Shuah Khan , Stephen Rothwell Subject: Re: [PATCH -manpage 1/2] fcntl.2: Update manpage with new memfd F_SEAL_FUTURE_WRITE seal To: "Joel Fernandes (Google)" , linux-kernel@vger.kernel.org References: <20190314214844.207430-1-joel@joelfernandes.org> <20190314214844.207430-2-joel@joelfernandes.org> From: "Michael Kerrisk (man-pages)" Message-ID: <11efee1c-0b65-2e51-0aa6-7f61fd2958c5@gmail.com> Date: Sat, 8 Feb 2020 12:58:07 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1 MIME-Version: 1.0 In-Reply-To: <20190314214844.207430-2-joel@joelfernandes.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Hello Joel, On 3/14/19 10:48 PM, Joel Fernandes (Google) wrote: > More details of the seal can be found in the LKML patch: > https://lore.kernel.org/lkml/20181120052137.74317-1-joel@joelfernandes.org/T/#t > > Signed-off-by: Joel Fernandes (Google) Thanks. Patch (finally) applied! Cheers, Michael > --- > man2/fcntl.2 | 15 +++++++++++++++ > 1 file changed, 15 insertions(+) > > diff --git a/man2/fcntl.2 b/man2/fcntl.2 > index fce4f4c2b3bd..e01e2c075b5b 100644 > --- a/man2/fcntl.2 > +++ b/man2/fcntl.2 > @@ -1525,6 +1525,21 @@ Furthermore, if there are any asynchronous I/O operations > .RB ( io_submit (2)) > pending on the file, > all outstanding writes will be discarded. > +.TP > +.BR F_SEAL_FUTURE_WRITE > +If this seal is set, the contents of the file can be modified only from > +existing writeable mappings that were created prior to the seal being set. > +Any attempt to create a new writeable mapping on the memfd via > +.BR mmap (2) > +will fail with > +.BR EPERM. > +Also any attempts to write to the memfd via > +.BR write (2) > +will fail with > +.BR EPERM. > +This is useful in situations where existing writable mapped regions need to be > +kept intact while preventing any future writes. For example, to share a > +read-only memory buffer to other processes that only the sender can write to. > .\" > .SS File read/write hints > Write lifetime hints can be used to inform the kernel about the relative > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/