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Message-ID: <20261112144550.GJ5716@brightrain.aerifal.cx>
Date: Thu, 12 Nov 2026 09:45:53 -0500
From: Rich Felker <dalias@...c.org>
To: Alejandro Colomar <alx+linux-man@...nel.org>
Cc: Walter Harms <wharms@....de>,
"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
libc-alpha@...rceware.org, musl@...ts.openwall.com,
~hallyn/shadow@...ts.sr.ht
Subject: Re: Re: RFC: how to document musl-crypt failure ?
On Thu, Nov 12, 2026 at 03:37:11PM +0100, Alejandro Colomar wrote:
> Hi Rich,
>
> > Date: 2026-11-09 16:51:02+0100
> > From: Alejandro Colomar <alx+linux-man@...nel.org>
> >
> [...]
> > > I would not document future behavior, and would document existing
> > > behavior only as an implementation detail, not an application-facing
> > > contract. The only contract with the application is that crypt(P,H)
> > > will never return a string identical to H except on valid password (in
> > > the absence of theoretical hash collisions) regardless of the validity
> > > of P and H.
> >
> > Okay. Thank you very much! I'll work on a patch, and CC you when it's
> > ready.
>
> What do you think of this?
>
> diff --git i/man/man3/crypt.3 w/man/man3/crypt.3
> index e0a74b955b3e..4a75fac2f627 100644
> --- i/man/man3/crypt.3
> +++ w/man/man3/crypt.3
> @@ -259,6 +259,28 @@ .SS salt
> is equal to
> .I xxx
> otherwise.
> +.SH EXAMPLES
> +When comparing a password against an existing hash,
> +that hash (and no other) must be passed as
> +.IR salt .
> +If and only if
> +.I salt
> +compares equal
> +(see
> +.BR streq (3))
> +to the returned string,
> +the password matches the existing hash specified by
> +.IR salt .
> +.P
> +.in +4n
> +.EX
> +if (streq(H, crypt(P, H))) {
> + // good password
> +} else {
> + // bad password
> +}
> +.EE
> +.in
> .SH SEE ALSO
> .BR login (1),
> .BR passwd (1),
>
> which reads as:
>
> --- HEAD:man/man3/crypt.3
> +++ ./man/man3/crypt.3
> @@ -152,6 +152,19 @@ NOTES
> than 1000, 999999999 if xxx is greater than 999999999, and
> is equal to xxx otherwise.
>
> +EXAMPLES
> + When comparing a password against an existing hash, that
> + hash (and no other) must be passed as salt. If and only
> + if salt compares equal (see streq(3)) to the returned
> + string, the password matches the existing hash specified
> + by salt.
> +
> + if (streq(H, crypt(P, H))) {
> + // good password
> + } else {
> + // bad password
> + }
> +
> SEE ALSO
> login(1), passwd(1), encrypt(3), getpass(3), passwd(5)
Missing null return check.
Rich
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