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Ensure VERSION is NUL-terminated #213

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merged 1 commit into from
Dec 2, 2020
Merged

Ensure VERSION is NUL-terminated #213

merged 1 commit into from
Dec 2, 2020

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suve
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@suve suve commented Nov 30, 2020

The VERSION variable is referenced in two places inside dumb-init.c: in the print_help() function, and in parse_command(), when the --version option is passed:

void print_help(char *argv[]) {
    fprintf(stderr,
        "dumb-init v%s"
        "Usage: %s [option] command [[arg] ...]\n"
// (snip)
        "Full help is available online at https://github.com/Yelp/dumb-init\n",
        VERSION,
        argv[0]
    );
}

// inside parse_command()
    case 'V':
        fprintf(stderr, "dumb-init v%s", VERSION);
        exit(0);

In both of these cases, VERSION is used as an argument to fprintf(), to be formatted with %s. The man page for fprintf(3) says this about the %s conversion specifier:

The const char * argument is expected to be a pointer to an array of character type (pointer to a string). Characters from the array are written up to (but not including) a terminating null byte ('\0');

However, the way VERSION is currently generated, it does NOT contain the terminating null byte.

unsigned char VERSION[] = {
  0x31, 0x2e, 0x32, 0x2e, 0x32, 0x0a
};

This patch changes the way VERSION.h is generated, to ensure that VERSION is a NUL-terminated string.

@asottile
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asottile commented Nov 30, 2020 •
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a less intrusive patch would be to use VERSION_len:

        fprintf(stderr, "dumb-init v%.*s", VERSION_len, VERSION);

@suve
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suve commented Dec 1, 2020 •
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It's an interesting idea, though it has one drawback - if someone needs to add a new piece of code that uses VERSION, they'll have to remember that it's not NUL-terminated and requires special handling.

As a third option, how about keeping VERSION as unsigned char[], with an additional 0-byte?

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asottile commented Dec 1, 2020 •
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I would prefer the one line patch as I indicated, this tool has had very little maintenance and I doubt that a new use for VERSION will come up

@suve
Use "%*s" when printing VERSION
7bb2fef 
The "%s" conversion specifier expects a NUL-terminated string.
However, the VERSION variable does not contain a NUL-terminator,
so formatting it using "%s" may lead to printing whatever happens
to be in memory next to VERSION.

Using "%*s" allows to specify how many characters to print,
thus making sure we don't go off the array.
@suve suve force-pushed the ensure-VERSION-is-NUL-terminated branch from 040188d to 7bb2fef Compare December 1, 2020 19:05
@suve
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suve commented Dec 1, 2020

Sure. Rewrote the patch and force-pushed the branch.

asottile
asottile approved these changes Dec 1, 2020
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@chriskuehl chriskuehl merged commit 6a0fcbc into Yelp:master Dec 2, 2020
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Thanks for the fix!

I think I won't release a new version just for this fix since as far as I understand this isn't causing issues in practice (due to automatic null byte padding around the constant?) but let me know if I'm misunderstanding the impact.

@suve
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suve commented Dec 2, 2020

Well, the only reason I spotted and patched this is because it did, in fact, cause issues in practice. ;)

In the Fedora Linux distribution, we had issues with building the dumb-init package. It turned out that on the s390x architecture, while the program built fine, the test suite failed - on this particular arch, the VERSION variable was not immediately followed by a null byte, which in turn caused the "is help message ok?" test to fail. (rhbz1863456)

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Thanks for letting me know, I've released a v1.2.3 which contains this fix.

@suve suve mentioned this pull request Dec 3, 2020
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@suve @asottile @chriskuehl