Re: [PATCH 1/1] string: Add stpecpy(3)

[Alejandro Colomar <alx.manpages@gmail.com>]

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Hi Sam!

On 12/23/22 08:02, Sam James wrote:
> 
> 
>> On 22 Dec 2022, at 21:42, Alejandro Colomar via Libc-alpha <libc-alpha@sourceware.org> wrote:
>>
>> Glibc didn't provide any function that copies a string with truncation.
>>
>> It only provided strncpy(3) and stpncpy(3), which copy from a string
>> into a null-padded character sequence at the destination fixed-width
>> buffer, with truncation.
>>
>> Those old functions, which don't produce a string, have been misused for
>> a long time as a poor-man's replacement for strlcpy(3), but doing that
>> is a source of bugs, since it's hard to calculate the right size that
>> should be passed to the function, and it's also necessary to explicitly
>> terminate the buffer with a null byte.  Detecting truncation is yet
>> another problem.
>>
>> stpecpy(3), described in the new string_copying(7) manual page, is
>> similar to OpenBSD's strlcpy(3)/strlcat(3), but:
>>
>> -  It's simpler to implement.
>> -  It's faster.
>> -  It's simpler to detect truncation.
> 
> Given strlcpy and strlcat are in POSIX next and therefore bar
> some extraordinary event will be in glibc, I think we should
> probably wait until those two land, then see if there's still
> an appetite for stpecpy in glibc.

I disagree for the following reasons.

strlcpy(3)/strlcat(3) are designed to be _slow_, in exchange for added 
simplicity and safety.  That's what Theo told me about them.  They didn't care 
about performance.  The two performance issues are:

-  Traverse the entire input string, to make sure it's a string.  stpecpy(3) 
instead only reads what's necessary for the copy; it stops reading after truncation.

-  strlcat(3) finds the terminating null byte; that's something you already know 
where it is, with functions that return a useful pointer (mempcpy(3), stpcpy(3), 
and stpecpy(3)).

While there are many programs that may be fine with the OpenBSD functions, glibc 
should _also_ provide a way to do the operation with an optimal API.  And it's 
in line with glibc providing stpcpy(3) and mempcpy(3) as extensions (stpcpy(3) 
is now in POSIX).


The reason that triggered me wanting to add this function is seeing strncpy(3) 
used for a patch to some glibc internals themselves.  Using strlcpy(3)/cat(3) in 
glibc internals would be bad for performance; I would hope that glibc uses the 
optimal internals, even if it also provides slow functions for users.

There are probably more cases within existing code in glibc.  Just check the 
output of:

     $ grep -rn st.ncpy -A1 | grep -B1 " = '\\\\0'"


Moreover, in the Austin discussion for strlcpy(3)/cat(3), it was mentioned that 
strlcpy(3) has an interface identical to that of snprintf(3), and that "If we 
truly think that this is bad design, should we come up with a new version of 
snprintf() that also doesn't do this? I don't think so.".

Well, I do believe snprintf is also misdesigned, for the same reasons that the 
strlcpy(3) manual page states that you should use strlcpy(3) for catenating 
strings, but rather strlcat(3):

        To detect truncation, perhaps while building a pathname, something like
        the following might be used:

              char *dir, *file, pname[MAXPATHLEN];

              ...

              if (strlcpy(pname, dir, sizeof(pname)) >= sizeof(pname))
                      goto toolong;
              if (strlcat(pname, file, sizeof(pname)) >= sizeof(pname))
                      goto toolong;

        Since it is known how many  characters  were  copied  the  first  time,
        things can be sped up a bit by using a copy instead of an append:

              char *dir, *file, pname[MAXPATHLEN];
              size_t n;

              ...

              n = strlcpy(pname, dir, sizeof(pname));
              if (n >= sizeof(pname))
                      goto toolong;
              if (strlcpy(pname + n, file, sizeof(pname) ‐ n) >= sizeof(pname) ‐ n)
                      goto toolong;

        However,  one  may question the validity of such optimizations, as they
        defeat the whole purpose of strlcpy() and strlcat().  As  a  matter  of
        fact, the first version of this manual page got it wrong.

Guess what?  There's no 'cat' version of snprintf, so users are doomed to write 
buggy code when trying to use it to concatenate after some other string.  I've 
recently been investigating a lot about it, and found invocations of Undefined 
Behavior, and some milder cases of benign off-by-one (on the safe side, by luck) 
errors, in calls to snprintf(3) in several important projects:

-  NGINX Unit:
    -  Undefined Behavior:
       <https://github.com/nginx/unit/issues/795#issuecomment-1345400420>

    -  Wrong truncation detection:
       <https://github.com/nginx/unit/pull/734#discussion_r1043963527>
       <https://github.com/nginx/unit/issues/804>

-  shadow:
    -  off-by-one:
       <https://github.com/shadow-maint/shadow/pull/607>

    -  clever code that looks like a bug but it's not:
       <https://github.com/shadow-maint/shadow/issues/608>

Rather than adding some catenating variant of snprintf(3), I suggest adding a 
single function that has an interface similar to stpcpy(3) and mempcpy(3), and 
identical to stpecpy(3):  stpeprintf(3):

<http://www.alejandro-colomar.es/src/alx/alx/libstp.git/tree/src/stp/stpe/stpeprintf.c>

I implemented it in terms of vsnprintf(3), so I need to handle a return of -1, 
but if implemented from scratch in glibc, in could be written to not be limited 
to INT_MAX (although I wonder why anyone would want to copy more than INT_MAX as 
a formatted string).


Below is its manual page in libstp.

Cheers,

Alex
---

stpeprintf(3)              Library Functions Manual              stpeprintf(3)

NAME
        stpeprintf, vstpeprintf - create a formatted string with truncation

LIBRARY
        Stp string library (libstp, pkgconf ‐‐cflags ‐‐libs libstp)

SYNOPSIS
        #include <stp/stpe/stpeprintf.h>

        char *_Nullable stpeprintf(char *_Nullable dst, char end[0],
                                   const char *restrict fmt, ...);
        char *_Nullable vstpeprintf(char *_Nullable dst, char end[0],
                                   const char *restrict fmt, va_list ap);

DESCRIPTION
        These functions are almost identical to snprintf(3) and vsnprintf(3).

        The  destination  buffer  is limited by a pointer to its end —one after
        its last element— instead of a size.

        These  functions  can  be  chained  with  calls  to  stpeprintf(3)  and
        vstpeprintf(3).

RETURN VALUE
        NULL
               •  If this function failed (see ERRORS).
               •  If dst was NULL.

        end
               •  If this call truncated.
               •  If  dst  was equal to end (a previous call to these functions
                  truncated).

        dst + strlen(dst)
               On success, these functions return a pointer to the  terminating
               null byte.

ERRORS
        These functions may fail for any of the same reasons as vsnprintf(3).

ATTRIBUTES
        For  an  explanation  of  the  terms  used in this section, see attrib‐
        utes(7).
        ┌────────────────────────────────────────────┬───────────────┬─────────┐
        │Interface                                   │ Attribute     │ Value   │
        ├────────────────────────────────────────────┼───────────────┼─────────┤
        │stpeprintf(3), vstpeprintf(3)               │ Thread safety │ MT‐Safe │
        └────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS
        None.

EXAMPLES
        See stpecpy(3).

SEE ALSO
        stpecpy(3), string_copying(7)

libstp (unreleased)                 (date)                       stpeprintf(3)








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<http://www.alejandro-colomar.es/>

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