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uprobes: Document uprobe_register() vs uprobe_mmap() race

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Because the mind is treacherous and makes us forget we need to
write stuff down.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Anton Arapov <anton@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20120615154339.GA9591@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2012-06-15 17:43:39 +02:00 committed by Ingo Molnar
parent 7a5bfb66b0
commit c5784de2b3
1 changed files with 28 additions and 3 deletions
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31
kernel/events/uprobes.c
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@ -44,6 +44,23 @@ static DEFINE_SPINLOCK(uprobes_treelock); /* serialize rbtree access */
#define UPROBES_HASH_SZ 13
/*
* We need separate register/unregister and mmap/munmap lock hashes because
* of mmap_sem nesting.
*
* uprobe_register() needs to install probes on (potentially) all processes
* and thus needs to acquire multiple mmap_sems (consequtively, not
* concurrently), whereas uprobe_mmap() is called while holding mmap_sem
* for the particular process doing the mmap.
*
* uprobe_register()->register_for_each_vma() needs to drop/acquire mmap_sem
* because of lock order against i_mmap_mutex. This means there's a hole in
* the register vma iteration where a mmap() can happen.
*
* Thus uprobe_register() can race with uprobe_mmap() and we can try and
* install a probe where one is already installed.
*/
/* serialize (un)register */
static struct mutex uprobes_mutex[UPROBES_HASH_SZ];
@ -339,7 +356,9 @@ static int is_swbp_at_addr(struct mm_struct *mm, unsigned long vaddr)
int __weak set_swbp(struct arch_uprobe *auprobe, struct mm_struct *mm, unsigned long vaddr)
{
int result;
/*
* See the comment near uprobes_hash().
*/
result = is_swbp_at_addr(mm, vaddr);
if (result == 1)
return -EEXIST;
@ -845,6 +864,10 @@ static int register_for_each_vma(struct uprobe *uprobe, bool is_register)
if (is_register) {
err = install_breakpoint(uprobe, mm, vma, info->vaddr);
/*
* We can race against uprobe_mmap(), see the
* comment near uprobe_hash().
*/
if (err == -EEXIST)
err = 0;
} else {
@ -1054,8 +1077,10 @@ int uprobe_mmap(struct vm_area_struct *vma)
}
ret = install_breakpoint(uprobe, vma->vm_mm, vma, vaddr);
/* Ignore double add: */
/*
* We can race against uprobe_register(), see the
* comment near uprobe_hash().
*/
if (ret == -EEXIST) {
ret = 0;