[PATCH v1 1/2] xstrtol: Correctly handle an invalid base

[Alejandro Colomar <alx@kernel.org>]

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strtol(3) doesn't set the end pointer if the base is invalid.  This
allows a caller to differentiate between "invalid base" (what
strtoi(3bsd) calls EINVAL) and an "no digits seen" (what strtoi(3bsd)
calls ECANCELED) in systems that report EINVAL on no digits seen (POSIX
allows this).

	strtol("foo", &e, 0);
	strtol("0", &e, -1);

The former call will set e = nptr.
The latter will leave e untouched.

The caller has no other way to portably differentiate the calls.

The way to differentiate those, thus, is to initialize e = NULL, to
allow reading it after the call.

While doing this, change the behavior of this function to only set
*endptr if strtol(3) has set it, leaving it untouched otherwise.

Fixes: 034a18049cbc (2014-12-20, "assure: new module")
Fixes: 64ddc975e72c (2024-07-18, "xstrtol: document and stray less from strtol")
Cc: Paul Eggert <eggert@cs.ucla.edu>
Cc: Bruno Haible <bruno@clisp.org>
Cc: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Cc: Eli Schwartz <eschwartz93@gmail.com>
Cc: Sam James <sam@gentoo.org>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Cc: Michael Vetter <jubalh@iodoru.org>
Cc: <liba2i@lists.linux.dev>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
---
 lib/xstrtol.c | 35 +++++++++++++++++++++--------------
 1 file changed, 21 insertions(+), 14 deletions(-)

diff --git a/lib/xstrtol.c b/lib/xstrtol.c
index c3145171f3..592673557f 100644
--- a/lib/xstrtol.c
+++ b/lib/xstrtol.c
@@ -71,9 +71,7 @@ strtol_error
 __xstrtol (char const *nptr, char **endptr, int base,
            __strtol_t *val, char const *valid_suffixes)
 {
-  char *t_ptr;
-  char **p = endptr ? endptr : &t_ptr;
-  *p = (char *) nptr;
+  char *e = NULL;
 
   if (! TYPE_SIGNED (__strtol_t))
     {
@@ -82,14 +80,21 @@ __xstrtol (char const *nptr, char **endptr, int base,
       while (isspace (ch))
         ch = *++q;
       if (ch == '-')
-        return LONGINT_INVALID;
+        {
+          if (endptr)
+            *endptr = (char *) nptr;
+          return LONGINT_INVALID;
+        }
     }
 
   errno = 0;
-  __strtol_t tmp = __strtol (nptr, p, base);
+  __strtol_t tmp = __strtol (nptr, &e, base);
   strtol_error err = LONGINT_OK;
 
-  if (*p == nptr)
+  if (endptr && e)
+    *endptr = e;
+
+  if (e == nptr)
     {
       /* If there is no number but there is a valid suffix, assume the
          number is 1.  The string is invalid otherwise.  */
@@ -113,19 +118,19 @@ __xstrtol (char const *nptr, char **endptr, int base,
       return err;
     }
 
-  if (**p != '\0')
+  if (*e != '\0')
     {
       int xbase = 1024;
       int suffixes = 1;
       strtol_error overflow;
 
-      if (!strchr (valid_suffixes, **p))
+      if (!strchr (valid_suffixes, *e))
         {
           *val = tmp;
           return err | LONGINT_INVALID_SUFFIX_CHAR;
         }
 
-      switch (**p)
+      switch (*e)
         {
         case 'E': case 'G': case 'g': case 'k': case 'K': case 'M': case 'm':
         case 'P': case 'Q': case 'R': case 'T': case 't': case 'Y': case 'Z':
@@ -138,10 +143,10 @@ __xstrtol (char const *nptr, char **endptr, int base,
              power-of-1024.  */
 
           if (strchr (valid_suffixes, '0'))
-            switch (p[0][1])
+            switch (e[1])
               {
               case 'i':
-                if (p[0][2] == 'B')
+                if (e[2] == 'B')
                   suffixes += 2;
                 break;
 
@@ -153,7 +158,7 @@ __xstrtol (char const *nptr, char **endptr, int base,
               }
         }
 
-      switch (**p)
+      switch (*e)
         {
         case 'b':
           overflow = bkm_scale (&tmp, 512);
@@ -224,8 +229,10 @@ __xstrtol (char const *nptr, char **endptr, int base,
         }
 
       err |= overflow;
-      *p += suffixes;
-      if (**p)
+      e += suffixes;
+      if (endptr)
+        *endptr = e;
+      if (*e)
         err |= LONGINT_INVALID_SUFFIX_CHAR;
     }
 
-- 
2.45.2


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