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References: <20240410021833.work.750-kees@kernel.org> <20240410023155.2100422-1-keescook@chromium.org>
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From: Andy Shevchenko <andy.shevchenko@gmail.com>
Date: Wed, 10 Apr 2024 07:08:10 +0300
Message-ID: <CAHp75VeOQ+x3WgFeG89X=kGUo=w=ztkdBkA3_K6yy+2mWV83AA@mail.gmail.com>
Subject: Re: [PATCH 1/5] string.h: Introduce memtostr() and memtostr_pad()
To: Kees Cook <keescook@chromium.org>
Cc: "Martin K . Petersen" <martin.petersen@oracle.com>, Justin Stitt <justinstitt@google.com>, 
	Andy Shevchenko <andy@kernel.org>, linux-hardening@vger.kernel.org, 
	Charles Bertsch <cbertsch@cox.net>, Bart Van Assche <bvanassche@acm.org>, 
	Sathya Prakash <sathya.prakash@broadcom.com>, 
	Sreekanth Reddy <sreekanth.reddy@broadcom.com>, 
	Suganath Prabu Subramani <suganath-prabu.subramani@broadcom.com>, 
	"James E.J. Bottomley" <jejb@linux.ibm.com>, Kashyap Desai <kashyap.desai@broadcom.com>, 
	Sumit Saxena <sumit.saxena@broadcom.com>, Nilesh Javali <njavali@marvell.com>, 
	Andrew Morton <akpm@linux-foundation.org>, Himanshu Madhani <himanshu.madhani@oracle.com>, 
	linux-kernel@vger.kernel.org, MPT-FusionLinux.pdl@broadcom.com, 
	linux-scsi@vger.kernel.org, mpi3mr-linuxdrv.pdl@broadcom.com, 
	GR-QLogic-Storage-Upstream@marvell.com
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On Wed, Apr 10, 2024 at 5:31=E2=80=AFAM Kees Cook <keescook@chromium.org> w=
rote:
>
> Another ambiguous use of strncpy() is to copy from strings that may not
> be NUL-terminated. These cases depend on having the destination buffer
> be explicitly larger than the source buffer's maximum size, having
> the size of the copy exactly match the source buffer's maximum size,
> and for the destination buffer to get explicitly NUL terminated.
>
> This usually happens when parsing protocols or hardware character arrays
> that are not guaranteed to be NUL-terminated. The code pattern is
> effectively this:
>
>         char dest[sizeof(src) + 1];
>
>         strncpy(dest, src, sizeof(src));
>         dest[sizeof(dest) - 1] =3D '\0';
>
> In practice it usually looks like:
>
> struct from_hardware {
>         ...
>         char name[HW_NAME_SIZE] __nonstring;
>         ...
> };
>
>         struct from_hardware *p =3D ...;
>         char name[HW_NAME_SIZE + 1];
>
>         strncpy(name, p->name, HW_NAME_SIZE);
>         name[NW_NAME_SIZE] =3D '\0';
>
> This cannot be replaced with:
>
>         strscpy(name, p->name, sizeof(name));
>
> because p->name is smaller and not NUL-terminated, so FORTIFY will
> trigger when strnlen(p->name, sizeof(name)) is used. And it cannot be
> replaced with:
>
>         strscpy(name, p->name, sizeof(p->name));
>
> because then "name" may contain a 1 character early truncation of
> p->name.
>
> Provide an unambiguous interface for converting a maybe not-NUL-terminate=
d
> string to a NUL-terminated string, with compile-time buffer size checking
> so that it can never fail at runtime: memtostr() and memtostr_pad(). Also
> add KUnit tests for both.

Obvious question, why can't strscpy() be fixed for this corner case?

--=20
With Best Regards,
Andy Shevchenko