libxl: Do not pass NULL as gc_opt; introduce NOGC

In 25182:6c3345d7e9d9 the practice of passing NULL to gc-using memory
allocation functions was introduced.  However, the arrangements there
were not correct as committed, because the error handling and logging
depends on getting a ctx from the gc - so an allocation error would in
fact result in libxl dereferencing NULL.

Instead, provide a special dummy gc in the ctx, called `nogc_gc'.  It
is marked out specially by having alloc_maxsize==-1, which is
otherwise invalid.

Functions which need to actually look into the gc use the new test
function gc_is_real (whose purpose is mainly clarity of the code) to
check whether the gc is the dummy one, and do nothing if it is.  And
we provide a helper macro NOGC which uses the in-scope real gc to find
the ctx and hence the dummy gc (and which replaces the previous
#define NOGC NULL).

Change all callers which pass 0 or NULL to an allocation function to
use NOGC or &ctx->nogc_gc, as applicable in the context.

We add a comment near the definition of LIBXL_INIT_GC pointing out
that it isn't any more the only place a libxl__gc struct is
initialised, for the benefit of anyone changing the contents of gc's
in the future.

Also, actually document that libxl__ptr_add is legal with ptr==NULL,
and change a couple of calls not to check for NULL argument.

Reported-by: Bamvor Jian Zhang <bjzhang@suse.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Bamvor Jian Zhang <bjzhang@suse.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/tools/libxl/libxl_internal.c b/tools/libxl/libxl_internal.c
index 8139520b78..fbff7d06d9 100644
--- a/tools/libxl/libxl_internal.c
+++ b/tools/libxl/libxl_internal.c
@@ -30,11 +30,16 @@ void libxl__alloc_failed(libxl_ctx *ctx, const char *func,
 #undef L
 }
 
+static int gc_is_real(const libxl__gc *gc)
+{
+    return gc->alloc_maxsize >= 0;
+}
+
 void libxl__ptr_add(libxl__gc *gc, void *ptr)
 {
     int i;
 
-    if (!gc)
+    if (!gc_is_real(gc))
         return;
 
     if (!ptr)
@@ -66,6 +71,8 @@ void libxl__free_all(libxl__gc *gc)
     void *ptr;
     int i;
 
+    assert(gc_is_real(gc));
+
     for (i = 0; i < gc->alloc_maxsize; i++) {
         ptr = gc->alloc_ptrs[i];
         gc->alloc_ptrs[i] = NULL;
@@ -104,7 +111,7 @@ void *libxl__realloc(libxl__gc *gc, void *ptr, size_t new_size)
 
     if (ptr == NULL) {
         libxl__ptr_add(gc, new_ptr);
-    } else if (new_ptr != ptr && gc != NULL) {
+    } else if (new_ptr != ptr && gc_is_real(gc)) {
         for (i = 0; i < gc->alloc_maxsize; i++) {
             if (gc->alloc_ptrs[i] == ptr) {
                 gc->alloc_ptrs[i] = new_ptr;