From: "Gustavo A. R. Silva" <gustavoars@kernel.org>
To: Darren Hart <dvhart@infradead.org>,
Andy Shevchenko <andy@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
"x86@kernel.org H. Peter Anvin" <hpa@zytor.com>
Cc: platform-driver-x86@vger.kernel.org,
linux-kernel@vger.kernel.org,
"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
Kees Cook <keescook@chromium.org>
Subject: [PATCH] x86/uv/time: Replace one-element array and save heap space
Date: Mon, 18 May 2020 14:01:14 -0500 [thread overview]
Message-ID: <20200518190114.GA7757@embeddedor> (raw)
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
next reply other threads:[~2020-05-18 18:56 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-18 19:01 Gustavo A. R. Silva [this message]
2020-05-18 19:09 ` [PATCH] x86/uv/time: Replace one-element array and save heap space Joe Perches
2020-05-18 22:32 ` Gustavo A. R. Silva
2020-05-20 17:19 ` Kees Cook
2020-05-20 19:01 ` Joe Perches
2020-05-21 23:24 ` Gustavo A. R. Silva
2020-09-29 23:14 ` Gustavo A. R. Silva
2020-09-30 6:52 ` Thomas Gleixner
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