firmware: dmi_scan: Fix handling of empty DMI strings

The handling of empty DMI strings looks quite broken to me:
* Strings from 1 to 7 spaces are not considered empty.
* True empty DMI strings (string index set to 0) are not considered
  empty, and result in allocating a 0-char string.
* Strings with invalid index also result in allocating a 0-char
  string.
* Strings starting with 8 spaces are all considered empty, even if
  non-space characters follow (sounds like a weird thing to do, but
  I have actually seen occurrences of this in DMI tables before.)
* Strings which are considered empty are reported as 8 spaces,
  instead of being actually empty.

Some of these issues are the result of an off-by-one error in memcmp,
the rest is incorrect by design.

So let's get it square: missing strings and strings made of only
spaces, regardless of their length, should be treated as empty and
no memory should be allocated for them. All other strings are
non-empty and should be allocated.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Fixes: 79da4721117f ("x86: fix DMI out of memory problems")
Cc: Parag Warudkar <parag.warudkar@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

diff --git a/drivers/firmware/dmi_scan.c b/drivers/firmware/dmi_scan.c
index 8cd5db6691b2d9c83450a83b1007e6c11a9e99f1..a7072e7880eec55940fc9561d75a697100224588 100644 (file)
--- a/drivers/firmware/dmi_scan.c
+++ b/drivers/firmware/dmi_scan.c
@@ -18,7 +18,7 @@ EXPORT_SYMBOL_GPL(dmi_kobj);
  * of and an antecedent to, SMBIOS, which stands for System
  * Management BIOS.  See further: http://www.dmtf.org/standards
  */
-static const char dmi_empty_string[] = "        ";
+static const char dmi_empty_string[] = "";
 
 static u32 dmi_ver __initdata;
 static u32 dmi_len;
@@ -39,25 +39,21 @@ static int dmi_memdev_nr;
 static const char * __init dmi_string_nosave(const struct dmi_header *dm, u8 s)
 {
        const u8 *bp = ((u8 *) dm) + dm->length;
+       const u8 *nsp;
 
        if (s) {
-               s--;
-               while (s > 0 && *bp) {
+               while (--s > 0 && *bp)
                        bp += strlen(bp) + 1;
-                       s--;
-               }
-
-               if (*bp != 0) {
-                       size_t len = strlen(bp)+1;
-                       size_t cmp_len = len > 8 ? 8 : len;
 
-                       if (!memcmp(bp, dmi_empty_string, cmp_len))
-                               return dmi_empty_string;
+               /* Strings containing only spaces are considered empty */
+               nsp = bp;
+               while (*nsp == ' ')
+                       nsp++;
+               if (*nsp != '\0')
                        return bp;
-               }
        }
 
-       return "";
+       return dmi_empty_string;
 }
 
 static const char * __init dmi_string(const struct dmi_header *dm, u8 s)