[sirwhinesalot]

There's nothing wrong with simple usages of goto.

The strxcpy family on the other hand is complete garbage and should never be used for any reason. I'm horrified that they're used in the kernel at all. All of those functions (and every failed attempt at "fixing" them) should have been nuked from orbit.

[Ysraes]

This is the approach taken in git https://github.com/git/git/blob/master/banned.h

[rdtsc]

> There's nothing wrong with simple usages of goto

Indeed a like a few gotos here and there for doing cleanup toward the end of the function.

[sirwhinesalot]

Or to break out of nested loops. The problem is with unstructured goto spaghetti making the code impossible to follow without essentially running it in your head (or a debugger).

Goto + Switch (or the GCC computed goto extension) is also a wonderful way to implement state machines.

[laweijfmvo]

What's wrong with `strncpy`?

[i80and]

strncpy won't always write a trailing nul byte, causing out of bounds reads elsewhere. It's a nasty little fellow. See the warning at https://linux.die.net/man/3/strncpy

strlcpy() is better and what most people think strncpy() is, but still results in truncated strings if not used carefully which can also lead to big problems.

[sirwhinesalot]

Speaking of strlcpy, Linus has some colorful opinions on it:

> Note that we have so few 'strlcpy()' calls that we really should remove that horrid horrid interface. It's a buggy piece of sh*t. 'strlcpy()' is fundamentally unsafe BY DESIGN if you don't trust the source string - which is one of the alleged reasons to use it. --Linus

Maybe strscpy is finally the one true fixed design to fix them all. Personally I think the whole exercise is one of unbeliavable stupidity when the real solution is obvious: using proper string buffer types with length and capacity for any sort of string manipulation.

[jjav]

> the real solution is obvious

If it were obvious it would have been done already. Witness the many variants that try to make it better but don't.

> using proper string buffer types with length and capacity

Which you then can't pass to any other library. String management is very easy to solve within the boundaries of your own code. But you'll need to interact with existing code as well.

[sirwhinesalot]

> If it were obvious it would have been done already. Witness the many variants that try to make it better but don't.

Every other language with mutable strings, including C++, does it like that. It is obvious. The reason it is not done in C is not ignorance, it is laziness.

> Which you then can't pass to any other library. String management is very easy to solve within the boundaries of your own code. But you'll need to interact with existing code as well.

Ignoring the also obvious solution of just keeping a null terminator around (see: C++ std::string), you should only worry about it at the boundary with the other library.

Same as converting from utf-8 to utf-16 to talk to the Windows API for example.

[jjav]

> The reason it is not done in C is not ignorance, it is laziness.

Of course not. C has been around since the dawn of UNIX and the majority of important libraries at the OS level are written in it.

Compatibility with such a vast amount of code is a lot more important than anything else.

If it were so easy why do you think nobody has done it?

> Ignoring the also obvious solution of just keeping a null terminator around

That's not very useful for the general case. If your code relies on the extra metadata (length, size) being correct and you're passing that null-terminated buffer around to libraries outside your code, it won't be correct since nothing else is aware of it.

[lelanthran]

> you should only worry about it at the boundary with the other library.

If this was a mitigation, it would solve all problems with nul-terminated strings i.e. do strict and error-checked conversions to nul-terminated strings at all boundaries to the program, and then nul-terminated strings and len-specified strings are equivalently dangerous (or safe, depending on your perspective).

The problem is precisely that unsanitised input makes its way into the application, bypassing any checks.

[jandrese]

For me the "real" solution looks something like this:

    ssize_t strxcpy(char* restrict dst, const char* restrict src, ssize_t len)

Strxcpy copies the string from src to dst. The len parameter is the number of bytes available in the dst buffer. The dst buffer is always terminated with a null byte, so the maximum length of string that can be copied into it is len - 1. strxcpy returns the number of characters copied on success, but can return the following negative values:

    E_INVALID_PARAMETER: Ether dst or src are NULL or len < 1, no data was copied
    W_TRUNCATED: len - 1 bytes were copied but more characters were available in src.

strxcat would work similarly. I have not decided if the return value should include the terminating null or not.

[jjav]

How is this useful though? I mean yes, it is useful in avoiding the buffer overruns. But that's not the only consideration, you also want code that handles data correctly. This just truncates at buffer size so data is lost.

So, if you want the code to work correctly, you need to either check the return code and reallocate dst and call the copy again. But if you're going to do that might as well check src len and allocate dst correctly before calling it so it never fails. But if you're already doing that, you can call strcpy just fine and never have a problem.

[raverbashing]

Wow yeah this seems to summarize well the usual api flakiness and just shuffling of C

It seems people come with "one more improvement" that's broken in one way or the other

[jandrese]

The problem with strlcpy is the return value. You can be burned badly if you are using it to for example pull out a fixed chunk of string from a 10TB memory mapped file, especially if you're pulling out all of the 32 byte chunks from that huge file and you just wanted a function to stick the trailing 0 on the string and handle short reads gracefully.

It's even worse if you are using it because you don't fully trust the input string to be null terminated. Maybe you have reasons to be believe that it will be at least as long as you need, but can't trust that it is a real string. As a function that was theoretically written as "fix" for strncpy it is worse in some fundamental ways. At least strncpy is easy enough to make safe by always over-allocating your buffer by 1 byte and stuffing a 0 in the last byte.

[kevin_thibedeau]

strncpy() also zero pads the entire buffer. If it's significantly larger than the copied string you're wasting cycles on pointless move operations for normal, low-security string handling. This behavior is for filling in fixed length fields in data structures. It isn't suitable for general purpose string processing.

[Borg3]

#define strncpyz(d,s,l) *(strncpy(d,s,l)+(l))=0

Of course this one is unsafe for macro expansion. But well, its C :)

[teo_zero]

I'd rather put the final nul at d+l-1 than at d+l, so that l can be the size of d, not "one more than the size of d":

  strncpyz(buf,src,sizeof buf);

[spacechild1]

As others have already pointed out it, it doesn't guarantee that the result is null-terminated. But that's not the only problem! In addition, it always pads the remaining space with zeros:

    char buf[1000];
    strncpy(buf, "foo", sizeof(buf));

This writes 3 characters and 9997 zeros. It's probably not what you want 99% of the time.

[lelanthran]

It's not possible to use it safely unless you know that the source string fits in the destination buffer. Every strncpy must be followed by `dst[sizeof dst - 1] = 0`, and even if you do that you still have no idea if you truncated the source string, so you have to put in a further check.

    strncpy (dst, src, (sizeof dst) - 1);
    dst[(sizeof dst) - 1] = 0;
    int truncated = strlen (dst) - strlen (src);

Without the extra two lines after every strncpy, you're probably going to have a a hard to discover transient bug.

[actionfromafar]

if you really want to use standard C string functions, use instead:

    int ret = snprintf(dst, sizeof dst, "%s", src);
    if (ret >= n || ret < 0)
    {
        /* failed */
    }

or as a function:

    bool ya_strcpy(const char* s, char* d, size_t n)
    {
        int cp = snprintf(d, n, "%s", s);
        bool ok = cp >= 0 && cp < n;
        ok ? *s = *s : 0;
        return ok;
    }

[kazinator]

snprintf only returns negative if an "encoding error" occurs, which has to do with multi-byte characters.

I think for that to possibly happen, you have to be in a locale with some character encoding in effect and snprintf is asked to print some multi-byte sequence that is invalid for that encoding.

Thus, I suspect, if you don't call that "f...f...frob my C program" function known as setlocale, it will never happen.

[lelanthran]

> Thus, I suspect, if you don't call that "f...f...frob my C program" function known as setlocale, it will never happen.

Of all the footguns in a hosted C implementation, I believe setlocale (and locale in general) is so broken that even compilers and library developers can't workaround it to make it safe.

The only other unfixable C-standard footgun that comes close, I think, are the environment-reading-and-writing functions, but at least with those, worst-case is leaking a negligible amount of memory in normal usage, or using an old value even when a newer one is available.

[kazinator]

I see that in Glibc, snprintf goes to the same general _IO_vsprintf function, which has various ominous -1 returns.

I don't think I see anything that looks like the detection of a conversion error, but rather other reasons. I would have to follow the code in detail to convince myself that glibc's snprintf cannot return -1 under some obscure conditions.

Defending against that value is probably wise.

As far as C locale goes, come on, the design was basically cemented in more or less its current form in 1989 ANSI C. What the hell did anyone know about internationalizing applications in 1989.

[lelanthran]

I actually do use `snprintf()` and friends.

[aulin]

except no one does that return code check and worse they often use the return code to advance a pointer in concatenated strings

[jlokier]

`strncpy` is commonly misunderstood. It's name misleads people into thinking it's a safely-truncating version of `strcpy`. It's not.

I've seen a lot of code where people changed from `strcpy` to `strncpy` because they thought that was safety and security best practice. Even sometimes creating a new security vulnerability which wasn't there with `strcpy`.

`strncpy` does two unexpected things which lead to safety, security and performance issues, especially in large codebases where the destination buffers are passed to other code:

• `strncpy` does NOT zero-terminate the copied string if it limits the length.

Whatever is given the copied string in future is vulnerable to a buffer-read-overrun and junk characters appended to the string, unless the reader has specific knowledge of the buffer length and is strict about NOT treating it as a null-terminated string. That's unusual C, so it's rarely done correctly. It also doesn't show up in testing or normal use, if `strnlen` is "for safety" and nobody enters data that large.

• `strncpy` writes the entire destination buffer with zeros after the copied string.

Usually this isn't a safety and security problem, but it can be terrible for performace if large buffers are being used to ensure there's room for all likely input data.

I've seen these issues in large, commercial C code, with unfortunate effects:

The code had a security fault because under some circumstances, a password check would read characters after the end of a buffer due to lack of a zero-terminator, that authors over the years assumed would always be there.

A password change function could set the new password to something different than the user entered, so they couldn't login after.

The code was assumed to be "fast" because it was C, and avoided "slow" memory allocation and a string API when processing strings. It used preallocated char arrays all over the place to hold temporary strings and `strncpy` to "safely" copy. They were wrong: It would have run faster with a clean string API that did allocations (for multiple reasons, not just `strncpy`).

Those char arrays had the slight inconvenience of causing oddly mismatched string length limits in text fields all over the place. But it was worth it for performance, they thought. To avoid that being a real problem, buffers tended to be sized to be "larger" than any likely value, so buffer sizes like 256 or 1000, 10000 or other arbitrary lengths plucked at random depending on developer mood at the time, and mismatched between countless different places in the large codebase. `strncpy` was used to write to them.

Using `malloc`, or better a proper string object API, would have run much faster in real use, at the same time as being safer and cleaner code.

Even worse, sometimes strings would be appended in pieces, each time using `strncpy` with the remaining length of the destination buffer. That filled the destination with zeros repeatedly, for every few characters appended. Sometimes causing user-interactions that would take milliseconds if coded properly, to take minutes.

Ironically, even a slow scripting language like Python using ordinary string type would have probably run faster than the C application. (Also Python dictionaries would have been faster than the buggy C hash tables in that application which took O(n) lookup time, and SQLite database tables would have been faster, smaller and simpler than the slow and large C "optimised" data structures they used to store data).

[sirwhinesalot]

It doesn't guarantee that the output is null terminated. Big source of exploits.