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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Sat, 22 Jul 2023 19:02:29 -0700
Message-ID: <CAADnVQKJ+SzCEaXxpSKemJo8p0bCOGcoOv1NDsJMsTsMmJmiZQ@mail.gmail.com>
Subject: Re: bpf: bpf_probe_read_user_str() returns 0 for empty strings
To:     Bagas Sanjaya <bagasdotme@gmail.com>
Cc:     Ingo Molnar <mingo@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        "Steven Rostedt (Google)" <rostedt@goodmis.org>,
        Max Froehling <Maximilian.Froehling@gdata.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux BPF <bpf@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>
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On Sat, Jul 22, 2023 at 6:53=E2=80=AFPM Bagas Sanjaya <bagasdotme@gmail.com=
> 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(v=
oid *dst, __u32 size, const void *unsafe_ptr returns 0 when the source stri=
ng (void *unsafe_ptr) consists of a string containing only a single null-by=
te.
> >
> > 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[=
] =3D ""; However, this is not the case. The function returns 0 instead.
> >
> > For non-empty strings, it works as expected. For example, char myString=
[] =3D "abc"; returns 4.
> >
> > Steps to Reproduce:
> > * Write an eBPF program that calls bpf_probe_read_user_str(), using a u=
serspace 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[] =3D "";
> > char copy[5];
> > long ret =3D bpf_probe_read_kernel_str(copy, 5, empty);
> > * Compare the return value of bpf_probe_read_user_str() and bpf_probe_r=
ead_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 help=
er implementation calls strncpy_from_user(), which returns the length witho=
ut trailing 0. Hence this function returns 0 for an empty string.
> >
> > However, in line 192 (as of commit fdf0eaf11452d72945af31804e2a1048ee1b=
574c) 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 doe=
s 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: A=
dd
> 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=3D217679
>
> --
> An old man doll... just what I always wanted! - Clara
>