Re: [PATCH v4 1/3] random.{4,7}, getrandom.2: Adapt to Linux 5.6 changes

[Alejandro Colomar <alx@kernel.org>]

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Ping.  :)

On Mon, Nov 24, 2025 at 05:44:17PM +0100, Alejandro Colomar wrote:
> Hi Mingye,
> 
> On Mon, Nov 10, 2025 at 11:51:30AM +0800, Mingye Wang wrote:
> > On Mon, Nov 10, 2025 at 11:05 AM Mingye Wang <arthur200126@gmail.com> wrote:
> > >
> > > > Does /dev/urandom block when reading with read(2) before the pool is
> > > ready?  I assume it blocks.
> > >
> > > From what I've heard it does not. Working on other comments.
> > 
> [...]
> > 
> > I am sending only the revised version of the patch you commented on
> > (the one that touches three files, random.{4,7}, getrandom.2). You
> > might notice that this patch is much larger. This is because when
> > editing the table I noticed that there's really no reason to keep the
> > "Pool" column around for 5.6+, but removing it also felt off. In the
> > end I just made two separate tables, but with identical cell-width
> > settings.
> 
> Please split into more patches.  I think this patch could be broken into
> a set of many small patches, each of which does one thing.  5 or 10
> patches would be okay.
> 
> > Speaking of cell-width settings, the existing version was not filling
> > the entire screen, which contravenes what man-pages(7) says about
> > using lbx. Adding an "x" does make it look nicer too.
> > 
> > (I have no idea whether this is the fourth version, but it's
> > definitely after the third and five seems too many. In any case, I
> > will be incrementing this number in future patch submissions to keep
> > things navigable.)
> > 
> > Regards,
> > Mingye Wang (Artoria2e5)
> 
> Some review of v4:
> 
> 	diff --git a/man/man4/random.4 b/man/man4/random.4
> 	index 0a651b03f..071fc99ef 100644
> 	--- a/man/man4/random.4
> 	+++ b/man/man4/random.4
> 	@@ -56,17 +56,29 @@ .SH DESCRIPTION
> 	 .I /dev/random
> 	 instead.
> 	 .P
> 	-The
> 	+.\" commit 30c08efec8884fb106b8e57094baa51bb4c44e32
> 	+Since Linux 5.6,
> 	 .I /dev/random
> 	-device is a legacy interface which dates back to
> 	+is identical to
> 	+.IR /dev/urandom ,
> 	+except that it blocks during early boot.
> 	+A jitter-based seeding technique added in Linux 5.4 should help reduce
> 	+block time.
> 	+.\" commit 50ee7529ec4500c88f8664560770a7a1b65db72b
> 	+.P
> 	+The pre-Linux 5.6
> 	+.I /dev/random
> 	+device was a legacy interface which dates back to
> 
> I'd prefer 'Before Linux 5.6,' instead of 'The pre-Linux 5.6'.
> 
> 	 a time where the cryptographic primitives used in the implementation
> 	 of
> 	 .I /dev/urandom
> 	 were not widely trusted.
> 	-It will return random bytes only within the estimated number of
> 	-bits of fresh noise in the entropy pool, blocking if necessary.
> 	+It would return random bytes only within the estimated number of bits of fresh
> 	+noise in the entropy pool, blocking until additional environmental noise is
> 	+gathered.
> 	+This old
> 	 .I /dev/random
> 
> Please use semantic newlines.  See man-pages(7):
> 
> $ MANWIDTH=72 man man-pages | sed -n '/Use semantic newlines/,/^$/p'
>    Use semantic newlines
>      In the source of a manual page, new sentences should be started on
>      new lines, long sentences should be split  into  lines  at  clause
>      breaks  (commas,  semicolons, colons, and so on), and long clauses
>      should be split at phrase boundaries.  This convention,  sometimes
>      known as "semantic newlines", makes it easier to see the effect of
>      patches, which often operate at the level of individual sentences,
>      clauses, or phrases.
> 
> I'd write it as:
> 
> ```
> It would return random bytes
> only within the estimated number of bits of fresh noise
> in the entropy pool,
> blocking until additional environmental noise is gathered.
> ```
> 
> Same here:
> 
> 	@@ -121,7 +133,8 @@ .SH DESCRIPTION
> 	 .BR read (2)
> 	 from
> 	 .I /dev/random
> 	-will return at most 512 bytes
> 	+has the same maximum size since Linux 5.6. Between Linux 3.16 and 5.5,
> 	+the maximum size was 512 bytes
> 	 .\" SEC_XFER_SIZE in drivers/char/random.c
> 	 (340 bytes before Linux 2.6.12).
> 	 .P
> 
> Use .P instead of .PP here:
> 
> 	diff --git a/man/man7/random.7 b/man/man7/random.7
> 	index fda408d38..65e21a07e 100644
> 	--- a/man/man7/random.7
> 	+++ b/man/man7/random.7
> 	@@ -54,17 +54,16 @@ .SS Initialization of the entropy pool
> 	 When a sufficient number of random bits has been collected, the
> 	 entropy pool is considered to be initialized.
> 	 .SS Choice of random source
> 	-Unless you are doing long-term key generation (and most likely not even
> 	-then), you probably shouldn't be reading from the
> 	+Unless your program may run at early-boot, before the entropy pool
> 	+is initialized, there is no longer any palpable difference between
> 	 .I /dev/random
> 	-device or employing
> 	-.BR getrandom (2)
> 	-with the
> 	-.B GRND_RANDOM
> 	-flag.
> 	-Instead, either read from the
> 	+and
> 	 .I /dev/urandom
> 	-device or employ
> 	+since Linux 5.6 (see the table below).
> 	+.PP
> 	+On older kernels, either read from the
> 	+.I /dev/urandom
> 	+device or (especially if you are concerned with early boot) employ
> 	 .BR getrandom (2)
> 	 without the
> 	 .B GRND_RANDOM
> 
> 
> Have a lovely day!
> Alex
> 
> -- 
> <https://www.alejandro-colomar.es>
> Use port 80 (that is, <...:80/>).



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