Re: [PATCH mm] maccess: fix strncpy_from_user_nofault empty string handling

[Andrew Morton]

On Thu, 17 Apr 2025 16:28:08 +0100 Mykyta Yatsenko <mykyta.yatsenko5@xxxxxxxxx> wrote:

> strncpy_from_user_nofault should return the length of the copied string
> including the trailing NUL, but if the argument unsafe_addr points to
> an empty string ({'\0'}), the return value is 0.
> 
> This happens as strncpy_from_user copies terminal symbol into dst
> and returns 0 (as expected), but strncpy_from_user_nofault does not
> modify ret as it is not equal to count and not greater than 0, so 0 is
> returned, which contradicts the contract.

Looks right, I think.

But why do strncpy_from_user() and strncpy_from_user_nofault() have
different interfaces?

/**
 * strncpy_from_user: - Copy a NUL terminated string from userspace.
 * ...
 * On success, returns the length of the string (not including the trailing
 * NUL).

/**
 * strncpy_from_user_nofault: - Copy a NUL terminated string from unsafe user
 *				address.
 * ...
 * On success, returns the length of the string INCLUDING the trailing NUL.

This is surprising and I'm wondering what led us to do this?