Ushering out strlcpy()

Posted Sep 2, 2022 13:38 UTC (Fri) by mathstuf (subscriber, #69389)
In reply to: Ushering out strlcpy() by mtodorov
Parent article: Ushering out strlcpy()

> strlen (buf) should thereof always be done as strnlen (buf, sizeof (buf)). Beyond the end of the buffer there can be beasts, nasty animals and hidden pits.

The problem is that `sizeof (buf)` is not known in a lot of contexts (e.g., filenames passed in from userspace). At that point, you're starting to carry around the allocated size around and you may as well amortize the length calculation as well…and oh, what do you know, you have a C++ std::string with no relevant established API.

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Ushering out strlcpy()

Posted Sep 2, 2022 13:50 UTC (Fri) by mtodorov (guest, #158788) [Link]

> The problem is that `sizeof (buf)` is not known in a lot of contexts (e.g., filenames passed in from userspace). At that point, you're starting to carry around the allocated size around and you may as well amortize the length calculation as well…and oh, what do you know, you have a C++ std::string with no relevant established API.

No argument on that.
However, in most cases there seems to be an implementation-specific limit, like `strnlen (buf, MAXPATHLEN)`.
Attempting to process more than that which makes sense usually only brings a risk of buffer overflow anyway. IMHO.
Carefully crafted input could place the first NULL byte at the beginning of the next memory page that is not allocated and trigger SIGBUS. Probably difficult to exploit, but still possible?