preprocessor: Fix #line overflow check [PR97602]

The preprocessor check for overflow (of linenum_type = unsigned int)
when reading the line number in a #line directive is incomplete; it
checks "reg < reg_prev" which doesn't cover all cases where
multiplying by 10 overflowed.  Fix this by checking for overflow
before rather than after it occurs (using essentially the same logic
as used by e.g. glibc printf when reading width and precision values
from strings).

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

libcpp/
2020-11-27  Joseph Myers  <joseph@codesourcery.com>

	PR preprocessor/97602
	* directives.c (strtolinenum): Check for overflow before it
	occurs.  Correct comment.

gcc/testsuite/
2020-11-27  Joseph Myers  <joseph@codesourcery.com>

	PR preprocessor/97602
	* gcc.dg/cpp/line9.c, gcc.dg/cpp/line10.c: New tests.

1 files changed, 5 insertions, 5 deletions

diff --git a/libcpp/directives.c b/libcpp/directives.c
index fa66b5c..7511560 100644
--- a/libcpp/directives.c
+++ b/libcpp/directives.c
@@ -915,12 +915,11 @@ read_flag (cpp_reader *pfile, unsigned int last)
 /* Subroutine of do_line and do_linemarker.  Convert a number in STR,
    of length LEN, to binary; store it in NUMP, and return false if the
    number was well-formed, true if not. WRAPPED is set to true if the
-   number did not fit into 'unsigned long'.  */
+   number did not fit into 'linenum_type'.  */
 static bool
 strtolinenum (const uchar *str, size_t len, linenum_type *nump, bool *wrapped)
 {
   linenum_type reg = 0;
-  linenum_type reg_prev = 0;
 
   uchar c;
   *wrapped = false;
@@ -929,11 +928,12 @@ strtolinenum (const uchar *str, size_t len, linenum_type *nump, bool *wrapped)
       c = *str++;
       if (!ISDIGIT (c))
 	return true;
+      if (reg > ((linenum_type) -1) / 10)
+	*wrapped = true;
       reg *= 10;
-      reg += c - '0';
-      if (reg < reg_prev) 
+      if (reg > ((linenum_type) -1) - (c - '0'))
 	*wrapped = true;
-      reg_prev = reg;
+      reg += c - '0';
     }
   *nump = reg;
   return false;