[clang] [Clang][Sema] Add fortify warnings for fread, fwrite, and fgets (PR #204337)

[Radovan Božić via cfe-commits]

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@@ -120,6 +126,14 @@ void call_bcopy_bzero(void) {
   __builtin_bzero(dst, 11); // expected-warning {{'bzero' will always overflow; destination buffer has size 10, but size argument is 11}}
 }
 
+void call_fread_fwrite_fgets(FILE *fp) {
+  char src[4];
+  fread(src, 2, 3, fp); // expected-warning {{'fread' will always overflow; destination buffer has size 4, but size argument is 6}}
+  fwrite(src, 2, 3, fp); // expected-warning {{'fwrite' will always read past the end of the source buffer; source buffer has size 4, but the size is 6}}
+  fgets(src, 5, fp); // expected-warning {{'fgets' size argument is too large; destination buffer has size 4, but size argument is 5}}
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bozicrHT wrote:

> What happens if someone calls `fgets` with a negative value?
> 
> Example:
> 
> ```c
> fgets(src, -1, fp);
> ```
> 
> Do we correctly diagnose that? Please add a unit test checking that.

Unfortunately, I missed that case. I’ll add support for negative size arguments to `fgets`.

https://github.com/llvm/llvm-project/pull/204337