How to disable the warning about using deprecated gets in GCC?

Question

I'm running a CTF and I am currently writing a problem that exploits C's gets function. I understand that the function is deprecated and dangerous and I would never use it in any other circumstance. Unfortunately, gcc compiles my code and when I run the binary when the gets function is hit, I get a friendly error message:

warning: this program uses gets(), which is unsafe.

This would normally be great, because it warns you that gets is unsafe, but unfortunately, in my CTF, I think that this error message makes the problem a bit too easy. Do you know how I would go about disabling this warning? Thanks!

$ gcc --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
Apple clang version 11.0.3 (clang-1103.0.32.62)
Target: x86_64-apple-darwin19.4.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

@JonathanLeffler - I think it's a type of organized "wargames" hacking competition: https://ctftime.org/ctf-wtf/ — selbie, May 28 '20 at 05:39
Is there anything that prevents you from not using `gets`? You can write your own unsafe `mygets` based on `fgets` (about 3-4 lines of code), or based on `fgetc` (~5-6 lines of code). — Jabberwocky, May 28 '20 at 06:55
I just realized that your warning has nothing to do with GCC itself - So the question title seems to be misplaced. The warning you got is from macOS about executing a program which uses `gets()`. — RobertS supports Monica Cellio, May 28 '20 at 10:16
@Jabberwocky post this as answer since this is best solution for competition like CFT. Current answer with `#pragma` doesn't improve how easy problem can be spotted. — Marek R, May 28 '20 at 10:20

score 8 · Accepted Answer · edited Jun 20 '20 at 09:12

Note: I just realized that your question title seems to be misplaced - The warning you got is from macOS about executing a program which uses gets(). It has nothing to do with the compilation by using GCC.

:-/ Any way, I let my answer alive for reference.

Just as comment: I googled a bit about what you are looking for, but there seems to be no reliable way to disable this warning when executing the program. One suggested rebuilding /usr/lib/libSystem.B.dylib without any result or experience if it indeed works, but I personally think this a bit too extreme and even can be harmful. - I do not recommend this technique.

If you really want to create an exploit program, try to rebuild gets() by a costum-made function and name the function a bit different, like f.e. gets_c(). This should be a workaround to disable this warning from macOS.

Old answer (regarding GCC itself):

First of all, you seem to be using a C99 or C89/C90-compliant compiler or alternatively compile with std=c99 or std=c89/std=c90 option, because only compilers conform to standards preceding C11 warn about gets() being deprecated.

ISO/IEC removed the gets() function in C11. If you would compile with a C11 or newer standard-compliant compiler, you would get an error about the implicit declaration of gets() when using it in the code instead:

"error: implicit declaration of function 'gets'; did you mean 'fgets'? [-Werror=implicit-function-declaration]"

If you want to suppress the warning at compilation, use the -Wno-deprecated-declarations option at compiling to disable the diagnostic for deprecated declarations.

From the GCC online docs:

-Wno-deprecated-declarations

Do not warn about uses of functions, variables, and types marked as deprecated by using the deprecated attribute. (see Function Attributes, see Variable Attributes, see Type Attributes.)

Source: https://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Warning-Options.html

If you want to embed the suppression of the warning in your code use the approach used in David´s deleted answer implementing a suppression for -Wno-deprecated-declarations by using #pragma:

   char str[256];

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
    gets(str);
#pragma GCC diagnostic pop

Nice answer, edited since my answer is no longer helpful – David Ranieri May 28 '20 at 09:32 — David Ranieri, May 28 '20 at 09:32

How to disable the warning about using deprecated gets in GCC?

1 Answers1