[TUHS] printf (was: python)

Dan Cross crossd at gmail.com
Sat Aug 5 07:16:56 AEST 2023

On Fri, Aug 4, 2023 at 12:57 PM Alejandro Colomar
<alx.manpages at gmail.com> wrote:
> On 2023-08-04 18:06, Dan Cross wrote:
> > On Thu, Aug 3, 2023 at 7:55 PM Alejandro Colomar <alx.manpages at gmail.com> wrote:
> >> On 2023-08-03 23:29, Dan Cross wrote:
> >>> On Thu, Aug 3, 2023 at 2:05 PM Alejandro Colomar <alx.manpages at gmail.com> wrote:
> >>>> -  It is type-safe, with the right tools.
> >>>
> >>> No it's not, and it really can't be. True, there are linters that can
> >>> try to match up types _if_ the format string is a constant and all the
> >>> arguments are known at e.g. compile time, but C permits one to
> >>> construct the format string at run time (or just select between a
> >>> bunch of variants); the language gives you no tools to enforce type
> >>> safety in a meaningful way once you do that.
> >>
> >> Isn't a variable format string a security vulnerability?  Where do you
> >> need it?
> >
> > It _can_ be a security vulnerability, but it doesn't necessarily
> > _need_ to be. If one is careful in how one constructs it, such things
> > can be very safe indeed.
> >
> > As to where one needs it, there are examples like `vsyslog()`,
> I guessed you'd mention v*() formatting functions, as that's the only
> case where a variable format string is indeed necessary (or kind of).

I think you are conflating "necessary" with "possible."

> I'll simplify your example to vwarnx(3), from the BSDs, which does less
> job, but has a similar API regarding our discussion.
> I'm not sure if you meant vsyslog() uses or its implementation, but
> I'll cover both (but for vwarnx(3)).
> Uses:
> This function (and all v*() functions) will be used to implement a
> wrapper variadic function, like for example warnx(3).  It's there, in
> the variadic function, where the string /must be/ a literal, and where

No, the format string does not need to be a literal at all: it can be
constructed at runtime. Is that a good idea? Perhaps not. Is it
possible? Yes. Can the compiler type-check it in that case? No, it
cannot (since it hasn't been constructed at compile time).  Consider
this program:

: chandra; cat warn.c
#include <err.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

        char buf[1024];

        strlcpy(buf, "%s ", sizeof(buf));
        strlcat(buf, "%s ", sizeof(buf));
        strlcat(buf, "%d", sizeof(buf));

        warnx(buf, "Hello", "World", 42);

        return EXIT_SUCCESS;
: chandra; cc -Wall -Werror -o warn warn.c
: chandra; ./warn
warn: Hello World 42
: chandra;

That's a perfectly legal C program, even if it is a silly one. "Don't
do that" isn't a statement about the language, it's a statement about
programmer practice, which is the point.

> the arguments are checked.  There's never a good reason to use a
> non-literal there (AFAIK),

I believe that you believe that. You may even be right. However,
that's not how the language works.

> and there are compiler warnings and linters
> to enforce that.  Since those args have been previously checked, you
> should just pass the va_list pristine to other formatting functions.

I'm afraid that this reasonable advice misses the point: there's
nothing in the language that says you _have_ to do it this way. Some
tools may _help_, but they cannot cover all (reasonable) situations.

Here again `syslog()` is an interesting example, as it supports the
`%m` formatting verb. _An_ implementation of this may work by
interpreting the format string and constructing a new one,
substituting `strerror(errno)` whenever it hits "%m" and then using
`snprintf` (or equivalent) to create the file string that is sent to
`syslogd`. You may argue that programmers should only pass constant
strings (left deliberately vague since there are reasonable cases
where named string constants may be passed as a format string argument
in lieu of a literal) that can be checked by clang and gcc, but again,
nothing in the language _requires_ that, but the implementation of
`vsyslog` that actually implements that logic has no way of knowing
that its caller has done this correctly.

Similarly, someone may choose to implement a templating language that
converts a custom format to a new format string, but assumes that the
arguments are in a `va_list` or similar. Bad idea? Probably. Legal in
C? Yes.

> Then, as long as libc doesn't have bugs, you're fine.

That's a tall order.

> In the implementation of a v*() function:
> Do /not/ touch the va_list.  Just pass it to the next function.  Of
> course, in the end, libc will have to iterate over it and do the job,
> but that's not the typical programmer's problem.  Here's the libbsd
> implementation of vwarnx(3), which does exactly that: no messing with
> the va_list.
> $ grepc vwarnx
> ./include/bsd/err.h:63:
> void vwarnx(const char *format, va_list ap)
>         __printflike(1, 0);
> ./src/err.c:97:
> void
> vwarnx(const char *format, va_list ap)
> {
>         fprintf(stderr, "%s: ", getprogname());
>         if (format)
>                 vfprintf(stderr, format, ap);
>         fprintf(stderr, "\n");
> }
> Just put a [[gnu::format(printf)]] in the outermost wrapper, which
> should be using a string literal, and you'll be fine.

Using a number of extensions aside here, again, that's just (sadly)
not how the language works.

> > but
> > that's almost besides the point, which is that given that you _can_ do
> > things like that, the language can't really save you by type-checking
> > the arguments to printf; and once varargs are in the mix? Forget about
> > it.
> Not really.  You can do that _only_ if you really want.

Yes, that's the point: if we're talking about language-level
guarantees, the language can't help you here. It can try, and it can
hit a lot of really useful cases, but not all. By contrast, formatting
in Go and Rust is type-safe by construction.

> If you want to
> not be able, you can "drop privileges" by adding a few flags to your
> compiler, such as -Werror=format-security -Werror=format-nonliteral,
> and add a bunch of linters to your build system for more redundancy,
> and voila, your project is now safe.

Provided that you use a compiler that provides those options, or that
those linters are viable in your codebase. ;-)

        - Dan C.

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