Messages in this thread Patch in this message | | | Subject | Re: [PATCH 1/3] uprobes: install_breakpoint() should fail if is_swbp_insn() == T | From | Peter Zijlstra <> | Date | Wed, 30 May 2012 20:21:01 +0200 |
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On Wed, 2012-05-30 at 23:34 +0530, Srikar Dronamraju wrote: > Nit: {,un}egister_uprobe should have been uprobe_{,un}register at > couple of places.
Ah indeed.. very silly.. hopefully last version.
--- Subject: uprobe: 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> --- kernel/events/uprobes.c | 29 ++++++++++++++++++++++++++++- 1 file changed, 28 insertions(+), 1 deletion(-)
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c index 985be4d..fd6fb30 100644 --- a/kernel/events/uprobes.c +++ b/kernel/events/uprobes.c @@ -45,6 +45,23 @@ 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]; @@ -356,6 +373,9 @@ int __weak set_swbp(struct arch_uprobe *auprobe, struct mm_struct *mm, unsigned { int result; + /* + * See the comment near uprobes_hash(). + */ result = is_swbp_at_addr(mm, vaddr); if (result == 1) return -EEXIST; @@ -870,6 +890,10 @@ static int register_for_each_vma(struct uprobe *uprobe, bool is_register) up_read(&mm->mmap_sem); mmput(mm); if (is_register) { + /* + * We can race against uprobe_mmap() see the comment + * near uprobe_hash(). + */ if (ret && ret == -EEXIST) ret = 0; if (ret) @@ -1080,7 +1104,10 @@ 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;
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