Re: [PATCH] x86/uv/time: Replace one-element array and save heap space

Gustavo A. R. Silva
2020-09-29T23:55:29Z

Hi all,

Who can take this?

Thanks
--
Gustavo

On 5/21/20 18:24, Gustavo A. R. Silva wrote:
> [+CC John Stultz <john.stultz@linaro.org> and +Kees' Reviewed-by tag]
>
> Reviewed-by: Kees Cook <keescook@chromium.org>
>
> On Mon, May 18, 2020 at 02:01:14PM -0500, Gustavo A. R. Silva wrote:
>> The current codebase makes use of one-element arrays in the following
>> form:
>>
>> struct something {
>> int length;
>> u8 data[1];
>> };
>>
>> struct something *instance;
>>
>> instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
>> instance->length = size;
>> memcpy(instance->data, source, size);
>>
>> but the preferred mechanism to declare variable-length types such as
>> these ones is a flexible array member[1][2], introduced in C99:
>>
>> struct foo {
>> int stuff;
>> struct boo array[];
>> };
>>
>> By making use of the mechanism above, we will get a compiler warning
>> in case the flexible array does not occur last in the structure, which
>> will help us prevent some kind of undefined behavior bugs from being
>> inadvertently introduced[3] to the codebase from now on. So, replace
>> the one-element array with a flexible-array member.
>>
>> Also, make use of the new struct_size() helper to properly calculate the
>> total size needed to allocate dynamic memory for struct uv_rtc_timer_head.
>> Notice that, due to the use of a one-element array, space for an extra
>> struct cpu:
>>
>> struct {
>> int lcpu; /* systemwide logical cpu number */
>> u64 expires; /* next timer expiration for this cpu */
>> } cpu[1]
>>
>> was being allocated at the moment of applying the sizeof operator to
>> struct uv_rtc_timer_head in the call to kmalloc_node() at line 159:
>>
>> 159 head = kmalloc_node(sizeof(struct uv_rtc_timer_head) +
>> 160 (uv_blade_nr_possible_cpus(bid) *
>> 161 2 * sizeof(u64)),
>> 162 GFP_KERNEL, nid);
>>
>> but that extra cpu[] was never actually being accessed due to the
>> following piece of code at line 168:
>>
>> 168 head->ncpus = uv_blade_nr_possible_cpus(bid);
>>
>> and the piece of code at line 187:
>>
>> 187 for (c = 0; c < head->ncpus; c++) {
>> 188 u64 exp = head->cpu[c].expires;
>> 189 if (exp < lowest) {
>> 190 bcpu = c;
>> 191 lowest = exp;
>> 192 }
>> 193 }
>>
>> so heap space was being wasted.
>>
>> Another thing important to notice is that through the use of the
>> struct_size() helper, code at line 161:
>>
>> 161 2 * sizeof(u64)),
>>
>> is changed to now be the actual size of struct cpu; see
>> sizeof(*(p)->member) at include/linux/overflow.h:314:
>>
>> 314 #define struct_size(p, member, n) \
>> 315 __ab_c_size(n, \
>> 316 sizeof(*(p)->member) + __must_be_array((p)->member),\
>> 317 sizeof(*(p)))
>>
>> As a side note, the original developer could have implemented code at line
>> 161: 2 * sizeof(64) as follows:
>>
>> sizeof(*head->cpu)
>>
>> This issue has been out there since 2009.
>>
>> This issue was found with the help of Coccinelle and fixed _manually_.
>>
>> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
>> [2] https://github.com/KSPP/linux/issues/21
>> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>>
>> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
>> ---
>> arch/x86/platform/uv/uv_time.c | 7 +++----
>> 1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/platform/uv/uv_time.c b/arch/x86/platform/uv/uv_time.c
>> index 7af31b245636..993a8ae6fdfb 100644
>> --- a/arch/x86/platform/uv/uv_time.c
>> +++ b/arch/x86/platform/uv/uv_time.c
>> @@ -52,7 +52,7 @@ struct uv_rtc_timer_head {
>> struct {
>> int lcpu; /* systemwide logical cpu number */
>> u64 expires; /* next timer expiration for this cpu */
>> - } cpu[1];
>> + } cpu[];
>> };
>>
>> /*
>> @@ -156,9 +156,8 @@ static __init int uv_rtc_allocate_timers(void)
>> struct uv_rtc_timer_head *head = blade_info[bid];
>>
>> if (!head) {
>> - head = kmalloc_node(sizeof(struct uv_rtc_timer_head) +
>> - (uv_blade_nr_possible_cpus(bid) *
>> - 2 * sizeof(u64)),
>> + head = kmalloc_node(struct_size(head, cpu,
>> + uv_blade_nr_possible_cpus(bid)),
>> GFP_KERNEL, nid);
>> if (!head) {
>> uv_rtc_deallocate_timers();
>> --
>> 2.26.2
>>