x86-linux: sys_set_thread_area: don't allocate GDT entry number zero,

and reject attempts to use it.  This is because the hardware does not
allow entry zero to be used, and apparently doing so confuses some
code (perhaps Windows apps running on Wine).  Derived from a patch by
John Reiser.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@10251 a5019735-40e9-0310-863c-91ae7b9d1cf9

1 files changed, 7 insertions, 3 deletions

diff --git a/coregrind/m_syswrap/syswrap-x86-linux.c b/coregrind/m_syswrap/syswrap-x86-linux.c
index e16b84488..af3a61f95 100644
--- a/coregrind/m_syswrap/syswrap-x86-linux.c
+++ b/coregrind/m_syswrap/syswrap-x86-linux.c
@@ -647,8 +647,11 @@ static SysRes sys_set_thread_area ( ThreadId tid, vki_modify_ldt_t* info )
    idx = info->entry_number;
 
    if (idx == -1) {
-      /* Find and use the first free entry. */
-      for (idx = 0; idx < VEX_GUEST_X86_GDT_NENT; idx++) {
+      /* Find and use the first free entry.  Don't allocate entry
+         zero, because the hardware will never do that, and apparently
+         doing so confuses some code (perhaps stuff running on
+         Wine). */
+      for (idx = 1; idx < VEX_GUEST_X86_GDT_NENT; idx++) {
          if (gdt[idx].LdtEnt.Words.word1 == 0 
              && gdt[idx].LdtEnt.Words.word2 == 0)
             break;
@@ -656,7 +659,8 @@ static SysRes sys_set_thread_area ( ThreadId tid, vki_modify_ldt_t* info )
 
       if (idx == VEX_GUEST_X86_GDT_NENT)
          return VG_(mk_SysRes_Error)( VKI_ESRCH );
-   } else if (idx < 0 || idx >= VEX_GUEST_X86_GDT_NENT) {
+   } else if (idx < 0 || idx == 0 || idx >= VEX_GUEST_X86_GDT_NENT) {
+      /* Similarly, reject attempts to use GDT[0]. */
       return VG_(mk_SysRes_Error)( VKI_EINVAL );
    }