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Subject[PATCHv4 16/28] x86/vdso: Allocate timens vdso
Date
As it has been discussed on timens RFC, adding a new conditional branch
`if (inside_time_ns)` on VDSO for all processes is undesirable.
It will add a penalty for everybody as branch predictor may mispredict
the jump. Also there are instruction cache lines wasted on cmp/jmp.

Those effects of introducing time namespace are very much unwanted
having in mind how much work have been spent on micro-optimisation
vdso code.

The propose is to allocate a second vdso code with dynamically
patched out (disabled by static_branch) timens code on boot time.

Allocate another vdso and copy original code.

Co-developed-by: Andrei Vagin <avagin@openvz.org>
Signed-off-by: Andrei Vagin <avagin@openvz.org>
Signed-off-by: Dmitry Safonov <dima@arista.com>
---
arch/x86/entry/vdso/vdso2c.h | 2 +-
arch/x86/entry/vdso/vma.c | 113 +++++++++++++++++++++++++++++++++--
arch/x86/include/asm/vdso.h | 9 +--
3 files changed, 114 insertions(+), 10 deletions(-)

diff --git a/arch/x86/entry/vdso/vdso2c.h b/arch/x86/entry/vdso/vdso2c.h
index 7556bb70ed8b..885b988aea19 100644
--- a/arch/x86/entry/vdso/vdso2c.h
+++ b/arch/x86/entry/vdso/vdso2c.h
@@ -157,7 +157,7 @@ static void BITSFUNC(go)(void *raw_addr, size_t raw_len,
}
fprintf(outfile, "\n};\n\n");

- fprintf(outfile, "const struct vdso_image %s = {\n", image_name);
+ fprintf(outfile, "struct vdso_image %s __ro_after_init = {\n", image_name);
fprintf(outfile, "\t.text = raw_data,\n");
fprintf(outfile, "\t.size = %lu,\n", mapping_size);
if (alt_sec) {
diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c
index 8a7f4cfe1fad..cc06c6b70167 100644
--- a/arch/x86/entry/vdso/vma.c
+++ b/arch/x86/entry/vdso/vma.c
@@ -30,26 +30,128 @@
unsigned int __read_mostly vdso64_enabled = 1;
#endif

-void __init init_vdso_image(const struct vdso_image *image)
+void __init init_vdso_image(struct vdso_image *image)
{
BUG_ON(image->size % PAGE_SIZE != 0);

apply_alternatives((struct alt_instr *)(image->text + image->alt),
(struct alt_instr *)(image->text + image->alt +
image->alt_len));
+#ifdef CONFIG_TIME_NS
+ image->text_timens = vmalloc_32(image->size);
+ if (WARN_ON(image->text_timens == NULL))
+ return;
+
+ memcpy(image->text_timens, image->text, image->size);
+#endif
}

struct linux_binprm;

+#ifdef CONFIG_TIME_NS
+static inline struct timens_offsets *current_timens_offsets(void)
+{
+ return current->nsproxy->time_ns->offsets;
+}
+
+static int vdso_check_timens(struct vm_area_struct *vma, bool *in_timens)
+{
+ struct task_struct *tsk;
+
+ if (likely(vma->vm_mm == current->mm)) {
+ *in_timens = !!current_timens_offsets();
+ return 0;
+ }
+
+ /*
+ * .fault() handler can be called over remote process through
+ * interfaces like /proc/$pid/mem or process_vm_{readv,writev}()
+ * Considering such access to vdso as a slow-path.
+ */
+
+#ifdef CONFIG_MEMCG
+ rcu_read_lock();
+
+ tsk = rcu_dereference(vma->vm_mm->owner);
+ if (tsk) {
+ task_lock(tsk);
+ /*
+ * Shouldn't happen: nsproxy is unset in exit_mm().
+ * Before that exit_mm() holds mmap_sem to set (mm = NULL).
+ * It's impossible to have a fault in task without mm
+ * and mmap_sem is taken during the fault.
+ */
+ if (WARN_ON_ONCE(tsk->nsproxy == NULL)) {
+ task_unlock(tsk);
+ rcu_read_unlock();
+ return -EIO;
+ }
+ *in_timens = !!tsk->nsproxy->time_ns->offsets;
+ task_unlock(tsk);
+ rcu_read_unlock();
+ return 0;
+ }
+ rcu_read_unlock();
+#endif
+
+ read_lock(&tasklist_lock);
+ for_each_process(tsk) {
+ struct task_struct *c;
+
+ if (tsk->flags & PF_KTHREAD)
+ continue;
+ for_each_thread(tsk, c) {
+ if (c->mm == vma->vm_mm)
+ goto found;
+ if (c->mm)
+ break;
+ }
+ }
+ read_unlock(&tasklist_lock);
+ return -ESRCH;
+
+found:
+ task_lock(tsk);
+ read_unlock(&tasklist_lock);
+ *in_timens = !!tsk->nsproxy->time_ns->offsets;
+ task_unlock(tsk);
+
+ return 0;
+}
+#else /* CONFIG_TIME_NS */
+static inline int vdso_check_timens(struct vm_area_struct *vma, bool *in_timens)
+{
+ *in_timens = false;
+ return 0;
+}
+static inline struct timens_offsets *current_timens_offsets(void)
+{
+ return NULL;
+}
+#endif /* CONFIG_TIME_NS */
+
static vm_fault_t vdso_fault(const struct vm_special_mapping *sm,
struct vm_area_struct *vma, struct vm_fault *vmf)
{
const struct vdso_image *image = vma->vm_mm->context.vdso_image;
+ unsigned long offset = vmf->pgoff << PAGE_SHIFT;
+ bool in_timens;
+ int err;

if (!image || (vmf->pgoff << PAGE_SHIFT) >= image->size)
return VM_FAULT_SIGBUS;

- vmf->page = virt_to_page(image->text + (vmf->pgoff << PAGE_SHIFT));
+ err = vdso_check_timens(vma, &in_timens);
+ if (err)
+ return VM_FAULT_SIGBUS;
+
+ WARN_ON_ONCE(in_timens && !image->text_timens);
+
+ if (in_timens && image->text_timens)
+ vmf->page = vmalloc_to_page(image->text_timens + offset);
+ else
+ vmf->page = virt_to_page(image->text + offset);
+
get_page(vmf->page);
return 0;
}
@@ -138,13 +240,14 @@ static vm_fault_t vvar_fault(const struct vm_special_mapping *sm,
return vmf_insert_pfn(vma, vmf->address,
vmalloc_to_pfn(tsc_pg));
} else if (sym_offset == image->sym_timens_page) {
- struct time_namespace *ns = current->nsproxy->time_ns;
+ /* We can fault only in current context for VM_PFNMAP mapping */
+ struct timens_offsets *offsets = current_timens_offsets();
unsigned long pfn;

- if (!ns->offsets)
+ if (!offsets)
pfn = page_to_pfn(ZERO_PAGE(0));
else
- pfn = page_to_pfn(virt_to_page(ns->offsets));
+ pfn = page_to_pfn(virt_to_page(offsets));

return vmf_insert_pfn(vma, vmf->address, pfn);
}
diff --git a/arch/x86/include/asm/vdso.h b/arch/x86/include/asm/vdso.h
index 9d420c545607..03f468c63a24 100644
--- a/arch/x86/include/asm/vdso.h
+++ b/arch/x86/include/asm/vdso.h
@@ -12,6 +12,7 @@

struct vdso_image {
void *text;
+ void *text_timens;
unsigned long size; /* Always a multiple of PAGE_SIZE */

unsigned long alt, alt_len;
@@ -30,18 +31,18 @@ struct vdso_image {
};

#ifdef CONFIG_X86_64
-extern const struct vdso_image vdso_image_64;
+extern struct vdso_image vdso_image_64;
#endif

#ifdef CONFIG_X86_X32
-extern const struct vdso_image vdso_image_x32;
+extern struct vdso_image vdso_image_x32;
#endif

#if defined CONFIG_X86_32 || defined CONFIG_COMPAT
-extern const struct vdso_image vdso_image_32;
+extern struct vdso_image vdso_image_32;
#endif

-extern void __init init_vdso_image(const struct vdso_image *image);
+extern void __init init_vdso_image(struct vdso_image *image);

extern int map_vdso_once(const struct vdso_image *image, unsigned long addr);

--
2.22.0
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