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, 2 insertions, 5 deletions

diff --git a/tools/libxl/libxl_xshelp.c b/tools/libxl/libxl_xshelp.c
index 993f527c58..7fdf164c42 100644
--- a/tools/libxl/libxl_xshelp.c
+++ b/tools/libxl/libxl_xshelp.c
@@ -86,11 +86,8 @@ char * libxl__xs_read(libxl__gc *gc, xs_transaction_t t, const char *path)
     char *ptr;
 
     ptr = xs_read(ctx->xsh, t, path, NULL);
-    if (ptr != NULL) {
-        libxl__ptr_add(gc, ptr);
-        return ptr;
-    }
-    return 0;
+    libxl__ptr_add(gc, ptr);
+    return ptr;
 }
 
 char *libxl__xs_get_dompath(libxl__gc *gc, uint32_t domid)