external c functions incorrectly accepted as safe

[bitwave]

I played a bit, and this compiles:

using <stdio.h>::{printf, scanf};

export fn main() -> int {
    char foo[2];

    scanf("%s", foo);
    printf("hello unsound %s\n", foo);
    return 0;
}

this should be unsound for len(stdin) > 2 or 1 (depends on NUL-byte or not) ASAN finds this at runtime, but it feels unsound.

[bitwave]

Another example:

using <stdio.h>::{printf, scanf};

export fn main() -> int {
    char *foo = 0x1337;

    printf("hello unsound %s\n", foo);
    return 0;
}

This is even simpler and simply unsafe.

[aep]

thank you for the report. calls to libc functions cannot be proven, because they have no zz signatures. therefor the checker is just disabled on any expression containing an external C type

still unsure if

1. this is fine
2. or require to be wrapped in an unsafe {} block
3. or be done like typescript where you can specify additional signatures for plain old C code.

[bitwave]

If you focusing on safety, everything which is not proven or assumed explicitely, should be unsafe.

[peirick]

i would go with an explicit unsafe block.

[fogti]

It should at least emit a warning, especially for known-unsafe c functions (e.g. like scanf on strings, gets, etc.).

References:

• https://wiki.sei.cmu.edu/confluence/display/c/MSC24-C.+Do+not+use+deprecated+or+obsolescent+functions
• https://rules.sonarsource.com/c/RSPEC-1081
• https://security.web.cern.ch/security/recommendations/en/codetools/c.shtml

inherently vulnerable/unsafe C functions:

• gets
• getpw
• rewind, setbuf
• strcat, strcpy
• sprintf

[aep]

this is currently blocked because an unsafe block is a scope, so this is not possible:

unsafe {
  int i = some_c_function();
}
int x = i;

and initialization in unsafe breaks SMT.

int i;
unsafe {
   i = some_c_function();
}
int x = i; // uninitialized read access

i'll either need to make unsafe an expression instead of a scope

int i = unsafe {
   some_c_function()
};
int x = i;

which means you need to wrap every call in unsafe rather than an entire list of statements.

or make it a block but ignore scoping so this is valid but harder to understand:

unsafe {
  int i = some_c_function();
}
int x = i;

another option is forcing c calls through a macro

int i = @extern(some_c_function());
int x = i;

the macro can do additional checks in first class library code rather than in the compiler

[fogti]

Could it be possible to make unsafe an expression, and for the less often needed block-unsafe add an unsafe_block, which creates an own scope?

[aep]

unsafe is now available in expression position:

    int e = unsafe<int>(errno);