Hello Nathan,
On Thu 23-01-20 09:56:10, Nathan Lynch wrote:
Libor Pechacek [off-list ref] writes:
quoted
In KVM guests drmem structure is only zero initialized. Trying to
manipulate DLPAR parameters results in a crash in this environment.
I think this statement needs qualification. Unless I'm mistaken, this
happens only when you boot a guest without any hotpluggable memory
configured, and then try to add or remove memory.
Thanks for the review. The introductory statement can indeed be clearer.
[...]
quoted
diff --git a/arch/powerpc/platforms/pseries/hotplug-memory.c b/arch/powerpc/platforms/pseries/hotplug-memory.c
index c126b94d1943..4ea6af002e27 100644
--- a/arch/powerpc/platforms/pseries/hotplug-memory.c
+++ b/arch/powerpc/platforms/pseries/hotplug-memory.c
@@ -236,9 +236,9 @@ static int get_lmb_range(u32 drc_index, int n_lmbs,
if (!start)
return -EINVAL;
- end = &start[n_lmbs - 1];
+ end = &start[n_lmbs];
- last_lmb = &drmem_info->lmbs[drmem_info->n_lmbs - 1];
+ last_lmb = &drmem_info->lmbs[drmem_info->n_lmbs];
if (end > last_lmb)
return -EINVAL;
Is this not undefined behavior? I'd rather do this in a way that does
not involve forming out-of-bounds pointers.
Well, this is a tough question for the case when drmem_info->lmbs was not
allocated. Given that the array does not exist, what bounds are we talking
about?
My patch builds on the fact that NULL[0] is NULL and NULL < NULL is false.
Talking about a pointer to one past the last element of an non-existent array
is too much philosophy for me.
For the case when drmem_info->lmbs is allocated, last_lmb is a pointer to one
past the last element of the array as Michal mentioned.
Even if it's safe, naming that pointer "last_lmb" now actively hinders
understanding of the code; it should be named "limit" or something.
Good catch.
[...]
quoted hunk ↗ jump to hunk
1 file changed, 36 insertions(+), 7 deletions(-)
arch/powerpc/include/asm/drmem.h | 43 +++++++++++++++++++++++++++++++++-------
modified arch/powerpc/include/asm/drmem.h
@@ -20,19 +20,48 @@ struct drmem_lmb {
struct drmem_lmb_info {
struct drmem_lmb *lmbs;
- int n_lmbs;
+ unsigned int n_lmbs;
u32 lmb_size;
};
extern struct drmem_lmb_info *drmem_info;
-#define for_each_drmem_lmb_in_range(lmb, start, end) \
- for ((lmb) = (start); (lmb) <= (end); (lmb)++)
+static inline bool drmem_present(void)
+{
+ return drmem_info->lmbs != NULL;
+}
Yes, use of this test was also my first idea about the fix.
quoted hunk ↗ jump to hunk
+static inline struct drmem_lmb *drmem_lmb_index(unsigned int index)
+{
+ if (!drmem_present())
+ return NULL;
-#define for_each_drmem_lmb(lmb) \
- for_each_drmem_lmb_in_range((lmb), \
- &drmem_info->lmbs[0], \
- &drmem_info->lmbs[drmem_info->n_lmbs - 1])
+ if (WARN_ON(index >= drmem_info->n_lmbs))
+ return NULL;
Why is this WARN_ON needed?
quoted hunk ↗ jump to hunk
+
+ return &drmem_info->lmbs[index];
+}
+
+static inline struct drmem_lmb *drmem_first_lmb(void)
+{
+ return drmem_lmb_index(0);
+}
+
+static inline struct drmem_lmb *drmem_last_lmb(void)
+{
+ if (!drmem_present())
+ return NULL;
+
+ return drmem_lmb_index(drmem_info->n_lmbs - 1);
Is the unsigned integer wraparound intended in drmem_info->n_lmbs == 0 case?
quoted hunk ↗ jump to hunk
+}
+
+#define for_each_drmem_lmb(lmb) \
+ for ((lmb) = drmem_first_lmb(); \
drmem_first_lmb() is essentially a call to drmem_info->lmbs(0). What
happens if drmem_info->n_lmbs is zero and drmem_info->lmbs is not NULL?
quoted hunk ↗ jump to hunk
+ (lmb) != NULL && (lmb) <= drmem_last_lmb(); \
+ (lmb)++)
+
+#define for_each_drmem_lmb_in_range(lmb, start, end) \
+ for ((lmb) = (start); (lmb) <= (end); (lmb)++)
/*
* The of_drconf_cell_v1 struct defines the layout of the LMB data
After all, I don't mind how the bug will be fixed. As you can see, my
preference is towards simpler solutions.
In my opinion your solution special-cased drmem_info->lmbs == NULL and opened
the doorway to the combination of drmem_info->lmbs != NULL &&
!drmem_info->n_lmbs. Maybe the condition can never become true but the code
should IMHO be robust enough to handle it.
Thanks!
Libor
--
Libor Pechacek
SUSE Labs Remember to have fun...