Re: [Tinycc-devel] Use of uninitalized automatic variable in TCC when parsing “int f ( int ( )”

[Pascal Cuoq]

Update: I have found an input that is accepted by GCC, accepted by Clang, and that makes TCC use the variable n uninitialized in function post_type.

On 08 Mar 2019, at 20:06, Pascal Cuoq <address@hidden> wrote:

the simplest way to make this problem visible is to instrument the functions type_decl and post_type:

diff --git a/tccgen.c b/tccgen.c

index 87ec798..7fa6c72 100644

--- a/tccgen.c

+++ b/tccgen.c

@@ -4374,7 +4374,7 @@ static int post_type(CType *type, AttributeDef *ad, int storage, int td)

     Sym **plast, *s, *first;

     AttributeDef ad1;

     CType pt;

-

+    n = 0xf00f0011;

     if (tok == '(') {

         /* function type, or recursive declarator (return if so) */

         next();

@@ -4410,6 +4410,7 @@ static int post_type(CType *type, AttributeDef *ad, int storage, int td)

                 }

                 convert_parameter_type(&pt);

                 arg_size += (type_size(&pt, &align) + PTR_SIZE - 1) / PTR_SIZE;

+                if (n == 0xf00f0011) printf("using n uninitialized\n");

                 s = sym_push(n | SYM_FIELD, &pt, 0, 0);

                 *plast = s;

                 plast = &s->next;

@@ -4583,7 +4584,7 @@ static CType *type_decl(CType *type, AttributeDef *ad, int *v, int td)

            parse_attribute(ad);

            post = type_decl(type, ad, v, td);

            skip(')');

-       }

+       } else printf("*v left uninitialized\n");

     } else if (tok >= TOK_IDENT && (td & TYPE_DIRECT)) {

        /* type identifier */

        *v = tok;

The function post_type declares an automatic variable n and does not initialize it. Setting it to 0xf00f0011 allows to see that it has not been assigned when it is used later in this function (ored with SYM_FIELD and passed as argument to the function sym_push). When “using n uninitialized” is printed in the instrumented version of TCC, it means that n would have been used uninitialized in the uninstrumented version of the compiler.

I have not found any syntactically correct input that caused “*v left uninitialized” to be printed but not “using n uninitialized”, so a solution *might* be to make TCC error out at the point where I made it print out “*v left uninitialized”, but this is for someone with better understanding of the code than me to decide.

A better input for demonstrating the problem (valid C compilation unit) is as follows:

$ cat cr.i

int f(const char *());

$ clang -Wall -c cr.i

$ gcc -Wall -c cr.i

$ ./tcc -c cr.i

*v left uninitialized

using n uninitialized