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Re: [Tinycc-devel] Use of uninitalized automatic variable in TCC when p

From: Pascal Cuoq
Subject: Re: [Tinycc-devel] Use of uninitalized automatic variable in TCC when parsing “int f ( int ( )”
Date: Sat, 9 Mar 2019 17:18:17 +0000

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) */
@@ -4410,6 +4410,7 @@ static int post_type(CType *type, AttributeDef *ad, int storage, int td)
                 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)
            post = type_decl(type, ad, v, td);
-       }
+       } 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

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