[PATCH] server: Make sure pids/tids are multiples of four.

[Rémi Bernon]

See: https://devblogs.microsoft.com/oldnewthing/?p=23283

Street Fighter V unpacker relies on it when validating other processes
for its anti-debug checks, it uses (PID&0xfffffffc)>>2 as an array index
and then checks back indexes against PIDs, and terminates early if some
PIDs do not match.

Signed-off-by: Rémi Bernon 
---
 server/process.c | 31 ++++++++++++++++++-------------
 1 file changed, 18 insertions(+), 13 deletions(-)

diff --git a/server/process.c b/server/process.c
index 73984f363f59..211207ed03b6 100644
--- a/server/process.c
+++ b/server/process.c
@@ -339,21 +339,24 @@ static void kill_all_processes(void);

#define PTID_OFFSET 8  /* offset for first ptid value */

+static unsigned int index_from_ptid(unsigned int id) { return id / 4; }
+static unsigned int ptid_from_index(unsigned int index) { return index * 4; }
+
 /* allocate a new process or thread id */
 unsigned int alloc_ptid( void *ptr )
 {
     struct ptid_entry *entry;
-    unsigned int id;
+    unsigned int index;

if (used_ptid_entries < alloc_ptid_entries)
     {
-        id = used_ptid_entries + PTID_OFFSET;
+        index = used_ptid_entries + PTID_OFFSET;
         entry = &ptid_entries[used_ptid_entries++];
     }
     else if (next_free_ptid && num_free_ptids >= 256)
     {
-        id = next_free_ptid;
-        entry = &ptid_entries[id - PTID_OFFSET];
+        index = next_free_ptid;
+        entry = &ptid_entries[index - PTID_OFFSET];
         if (!(next_free_ptid = entry->next)) last_free_ptid = 0;
         num_free_ptids--;
     }
@@ -368,35 +371,37 @@ unsigned int alloc_ptid( void *ptr )
         }
         ptid_entries = entry;
         alloc_ptid_entries = count;
-        id = used_ptid_entries + PTID_OFFSET;
+        index = used_ptid_entries + PTID_OFFSET;
         entry = &ptid_entries[used_ptid_entries++];
     }

entry->ptr = ptr;
-    return id;
+    return ptid_from_index( index );
 }

/* free a process or thread id */
 void free_ptid( unsigned int id )
 {
-    struct ptid_entry *entry = &ptid_entries[id - PTID_OFFSET];
+    unsigned int index = index_from_ptid( id );
+    struct ptid_entry *entry = &ptid_entries[index - PTID_OFFSET];

entry->ptr  = NULL;
     entry->next = 0;

/* append to end of free list so that we don't reuse it too early */
-    if (last_free_ptid) ptid_entries[last_free_ptid - PTID_OFFSET].next = id;
-    else next_free_ptid = id;
-    last_free_ptid = id;
+    if (last_free_ptid) ptid_entries[last_free_ptid - PTID_OFFSET].next = index;
+    else next_free_ptid = index;
+    last_free_ptid = index;
     num_free_ptids++;
 }

/* retrieve the pointer corresponding to a process or thread id */
 void *get_ptid_entry( unsigned int id )
 {
-    if (id < PTID_OFFSET) return NULL;
-    if (id - PTID_OFFSET >= used_ptid_entries) return NULL;
-    return ptid_entries[id - PTID_OFFSET].ptr;
+    unsigned int index = index_from_ptid( id );
+    if (index < PTID_OFFSET) return NULL;
+    if (index - PTID_OFFSET >= used_ptid_entries) return NULL;
+    return ptid_entries[index - PTID_OFFSET].ptr;
 }

/* return the main thread of the process */