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Subject: Re: [PATCH v3] kdb: Replace deprecated strcpy() with strscpy() and
 memcpy()
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From: Thorsten Blum <thorsten.blum@linux.dev>
In-Reply-To: <CAD=FV=Udf3pZjhpPxEuHsFynP7GoHnZ7RG=NYZ2gLzm=E_4V4A@mail.gmail.com>
Date: Fri, 15 Aug 2025 12:48:02 +0200
Cc: Jason Wessel <jason.wessel@windriver.com>,
 Daniel Thompson <danielt@kernel.org>,
 Nir Lichtman <nir@lichtman.org>,
 Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
 Yuran Pereira <yuran.pereira@hotmail.com>,
 linux-hardening@vger.kernel.org,
 Daniel Thompson <daniel@riscstar.com>,
 kgdb-bugreport@lists.sourceforge.net,
 linux-kernel@vger.kernel.org
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References: <20250814220130.281187-2-thorsten.blum@linux.dev>
 <CAD=FV=Udf3pZjhpPxEuHsFynP7GoHnZ7RG=NYZ2gLzm=E_4V4A@mail.gmail.com>
To: Doug Anderson <dianders@chromium.org>
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Hi Doug,

On 15. Aug 2025, at 04:05, Doug Anderson wrote:
> Let's think about some test cases...
> 
> Old code:
> mp->usage = kdb_strdup(argv[2], GFP_KDB);
> if (mp->usage[0] == '"') {
>  strcpy(mp->usage, argv[2]+1);
>  mp->usage[strlen(mp->usage)-1] = '\0';
> }
> 
> New code:
> mp->usage = kdb_strdup(argv[2], GFP_KDB);
> if (mp->usage[0] == '"')
>  strscpy(mp->usage, argv[2] + 1, strlen(argv[2]) - 1);
> 
> Example string: argv[2] = "\"xyz\""
> 
> Old:
>  mp->usage = strdup("\"xyz\"")
>  mp->usage becomes "xyz\""
>  mp->usage becomes "xyz"
> 
> New:
>  mp->usage = strdup("\"xyz\"")
>  mp->usage becomes "xyz\""
>  mp->usage doesn't change (!)
> 
> To match old behavior, I think you'd need "strlen(argv[2]) - 2", right?

No, it should be "strlen(argv[2]) - 1" to match the old behavior.

In the new code, there are only two steps instead of three.

With your example source string "\"xyz\"" in argv[2]:

	strscpy(mp->usage, argv[2] + 1, strlen(argv[2]) - 1)

evaluates to:

	strscpy(mp->usage, "xyz\"", strlen("\"xyz\"") - 1)

strlen("\"xyz\"") is 5, so this becomes:

	strscpy(mp->usage, "xyz\"", 4)

Unlike strcpy(), strscpy() copies at most 'size - 1' characters and then
appends a NUL terminator. In the example, it copies only the first three
bytes (xyz) and then appends a NUL terminator, effectively replacing the
trailing quote. The result is "xyz", the same as before.

Thanks,
Thorsten