Re: bpf: bpf_probe_read_user_str() returns 0 for empty strings

[Alexei Starovoitov]

On Sat, Jul 22, 2023 at 6:53 PM Bagas Sanjaya <bagasdotme@xxxxxxxxx> wrote:
>
> Hi,
>
> I notice a bug report on Bugzilla [1]. Quoting from it:
>
> > Overview:
> >
> > From within eBPF, calling the helper function bpf_probe_read_user_str(void *dst, __u32 size, const void *unsafe_ptr returns 0 when the source string (void *unsafe_ptr) consists of a string containing only a single null-byte.
> >
> > This violates various functions documentations (the helper and various internal kernel functions), which all state:

Sounds like the bugzilla author believes it's a documentation issue.
If so, please encourage the author to send the patch to fix the doc.

> >
> >> On success, the strictly positive length of the output string,
> >> including the trailing NUL character. On error, a negative value.
> >
> > To me, this states that the function should return 1 for char myString[] = ""; However, this is not the case. The function returns 0 instead.
> >
> > For non-empty strings, it works as expected. For example, char myString[] = "abc"; returns 4.
> >
> > Steps to Reproduce:
> > * Write an eBPF program that calls bpf_probe_read_user_str(), using a userspace pointer pointing to an empty string.
> > * Store the result value of that function
> > * Do the same thing, but try out bpf_probe_read_kernel_str(), like this:
> > char empty[] = "";
> > char copy[5];
> > long ret = bpf_probe_read_kernel_str(copy, 5, empty);
> > * Compare the return value of bpf_probe_read_user_str() and bpf_probe_read_kernel_str()
> >
> > Expected Result:
> >
> > Both functions return 1 (because of the single NULL byte).
> >
> > Actual Result:
> >
> > bpf_probe_read_user_str() returns 0, while bpf_probe_read_kernel_str() returns 1.
> >
> > Additional Information:
> >
> > I believe I can see the bug on the current Linux kernel master branch.
> >
> > In the file/function mm/maccess.c::strncpy_from_user_nofault() the helper implementation calls strncpy_from_user(), which returns the length without trailing 0. Hence this function returns 0 for an empty string.
> >
> > However, in line 192 (as of commit fdf0eaf11452d72945af31804e2a1048ee1b574c) there is a check that only increments ret, if it is > 0. This appears to be the logic that adds the trailing null byte. Since the check only does this for a ret > 0, a ret of 0 remains at 0.
> >
> > This is a possible off-by-one error that might cause the behavior.
>
> See Bugzilla for the full thread.
>
> FYI, the culprit line is introduced by commit 3d7081822f7f9e ("uaccess: Add
> non-pagefault user-space read functions"). I Cc: culprit SoB so that they
> can look into this bug.
>
> Thanks.
>
> [1]: https://bugzilla.kernel.org/show_bug.cgi?id=217679
>
> --
> An old man doll... just what I always wanted! - Clara
>