uprobes: Document uprobe_register() vs uprobe_mmap() race
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>
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@ -44,6 +44,23 @@ static DEFINE_SPINLOCK(uprobes_treelock); /* serialize rbtree access */
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#define UPROBES_HASH_SZ 13
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#define UPROBES_HASH_SZ 13
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/*
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* We need separate register/unregister and mmap/munmap lock hashes because
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* of mmap_sem nesting.
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*
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* uprobe_register() needs to install probes on (potentially) all processes
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* and thus needs to acquire multiple mmap_sems (consequtively, not
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* concurrently), whereas uprobe_mmap() is called while holding mmap_sem
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* for the particular process doing the mmap.
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*
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* uprobe_register()->register_for_each_vma() needs to drop/acquire mmap_sem
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* because of lock order against i_mmap_mutex. This means there's a hole in
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* the register vma iteration where a mmap() can happen.
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*
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* Thus uprobe_register() can race with uprobe_mmap() and we can try and
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* install a probe where one is already installed.
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*/
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/* serialize (un)register */
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/* serialize (un)register */
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static struct mutex uprobes_mutex[UPROBES_HASH_SZ];
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static struct mutex uprobes_mutex[UPROBES_HASH_SZ];
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@ -339,7 +356,9 @@ out:
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int __weak set_swbp(struct arch_uprobe *auprobe, struct mm_struct *mm, unsigned long vaddr)
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int __weak set_swbp(struct arch_uprobe *auprobe, struct mm_struct *mm, unsigned long vaddr)
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{
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{
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int result;
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int result;
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/*
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* See the comment near uprobes_hash().
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*/
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result = is_swbp_at_addr(mm, vaddr);
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result = is_swbp_at_addr(mm, vaddr);
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if (result == 1)
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if (result == 1)
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return -EEXIST;
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return -EEXIST;
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@ -845,6 +864,10 @@ static int register_for_each_vma(struct uprobe *uprobe, bool is_register)
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if (is_register) {
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if (is_register) {
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err = install_breakpoint(uprobe, mm, vma, info->vaddr);
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err = install_breakpoint(uprobe, mm, vma, info->vaddr);
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/*
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* We can race against uprobe_mmap(), see the
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* comment near uprobe_hash().
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*/
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if (err == -EEXIST)
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if (err == -EEXIST)
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err = 0;
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err = 0;
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} else {
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} else {
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@ -1054,8 +1077,10 @@ int uprobe_mmap(struct vm_area_struct *vma)
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}
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}
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ret = install_breakpoint(uprobe, vma->vm_mm, vma, vaddr);
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ret = install_breakpoint(uprobe, vma->vm_mm, vma, vaddr);
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/*
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/* Ignore double add: */
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* We can race against uprobe_register(), see the
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* comment near uprobe_hash().
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*/
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if (ret == -EEXIST) {
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if (ret == -EEXIST) {
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ret = 0;
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ret = 0;
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