linux-kernel - Re: [PATCH v2 1/6] uaccess: Add copy_from_user

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Message-ID: <aWaIOT_o-99G-_r-@yury>
Date: Tue, 13 Jan 2026 13:00:25 -0500
From: Yury Norov <ynorov@...dia.com>
To: Fushuai Wang <fushuai.wang@...ux.dev>
Cc: tglx@...nel.org, peterz@...radead.org, mathieu.desnoyers@...icios.com,
	akpm@...ux-foundation.org, aliceryhl@...gle.com,
	yury.norov@...il.com, vmalik@...hat.com, kees@...nel.org,
	dave.hansen@...ux.intel.com, luto@...nel.org, mingo@...hat.com,
	bp@...en8.de, hpa@...or.com, rostedt@...dmis.org,
	mhiramat@...nel.org, brauner@...nel.org, jack@...e.cz,
	cyphar@...har.com, linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-trace-kernel@...r.kernel.org, wangfushuai@...du.com
Subject: Re: [PATCH v2 1/6] uaccess: Add copy_from_user_nul helper

On Mon, Jan 12, 2026 at 03:30:34PM +0800, Fushuai Wang wrote:
> From: Fushuai Wang <wangfushuai@...du.com>
> 
> Many places call copy_from_user() to copy a buffer from user space,
> and then manually add a NULL terminator to the destination buffer,
> e.g.:

6 is not many

> 
> 	if (copy_from_user(dest, src, len))
> 		return -EFAULT;
> 	dest[len] = '\0';
> 
> This is repetitive and error-prone. Add a copy_from_user_nul() helper to
> simplify such patterns. It copied n bytes from user space to kernel space,
> and NUL-terminates the destination buffer.
> 
> Signed-off-by: Fushuai Wang <wangfushuai@...du.com>

I checked the cases you've found, and all them clearly abuse
copy_from_user(). For example, #2 in tlbflush_write_file():

        if (copy_from_user(buf, user_buf, len))
                return -EFAULT;

        buf[len] = '\0';
        if (kstrtoint(buf, 0, &ceiling))
                return -EINVAL;

should be:

        len = strncpy_from_user(buf, user_buf, len);
        if (len < 0)
                return len;

        ret = kstrtoint(buf, 0, &ceiling);
        if (ret)
                return ret;

See, if you use the right API, you don't need this weird
copy_from_user_nul(). Also notice how nice the original version hides
possible ERANGE in kstrtoint().

Patches #3-5 in the series again copy strings with raw non-string API,
so should be converted to some flavor of strcpy().

#6 patches lib/kstrtox, which makes little sense because the whole
purpose of that library is to handle raw pieces of memory as valid
C strings. One would expect such patterns in library code, and I'd
prefer having them explicit.

I find copy_{from,to}_user_nul() useful for objects that must be
null-terminated, and may have \0 somewhere in the middle. Those are
not C strings. I suspect this isn't a popular format across the kernel. 

On the other hand, adding the _nul() version of copy_from_user() would
make an API abuse like above simpler, which is a bad thing.

Can you drop copy_from_user_nul() and submit a series that switches
string manipulations to the dedicated string functions?

Thanks,
Yury