The following commit has been merged into the perf/core branch of tip: Commit-ID: 84455e6923c79a37812930787aaa141e82afe315 Gitweb: https://git.kernel.org/tip/84455e6923c79a37812930787aaa141e82afe315 Author: Oleg Nesterov <oleg@xxxxxxxxxx> AuthorDate: Thu, 01 Aug 2024 15:27:09 +02:00 Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx> CommitterDate: Fri, 02 Aug 2024 11:30:30 +02:00 uprobes: document the usage of mm->mmap_lock The comment above uprobe_write_opcode() is wrong, unapply_uprobe() calls it under mmap_read_lock() and this is correct. And it is completely unclear why register_for_each_vma() takes mmap_lock for writing, add a comment to explain that mmap_write_lock() is needed to avoid the following race: - A task T hits the bp installed by uprobe and calls find_active_uprobe() - uprobe_unregister() removes this uprobe/bp - T calls find_uprobe() which returns NULL - another uprobe_register() installs the bp at the same address - T calls is_trap_at_addr() which returns true - T returns to handle_swbp() and gets SIGTRAP. Reported-by: Andrii Nakryiko <andrii@xxxxxxxxxx> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Reviewed-by: Jiri Olsa <jolsa@xxxxxxxxxx> Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx> Acked-by: "Masami Hiramatsu (Google)" <mhiramat@xxxxxxxxxx> Link: https://lore.kernel.org/r/20240801132709.GA8780@xxxxxxxxxx --- kernel/events/uprobes.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c index f69ecd3..2d1457e 100644 --- a/kernel/events/uprobes.c +++ b/kernel/events/uprobes.c @@ -453,7 +453,7 @@ static int update_ref_ctr(struct uprobe *uprobe, struct mm_struct *mm, * @vaddr: the virtual address to store the opcode. * @opcode: opcode to be written at @vaddr. * - * Called with mm->mmap_lock held for write. + * Called with mm->mmap_lock held for read or write. * Return 0 (success) or a negative errno. */ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm, @@ -1046,7 +1046,13 @@ register_for_each_vma(struct uprobe *uprobe, struct uprobe_consumer *new) if (err && is_register) goto free; - + /* + * We take mmap_lock for writing to avoid the race with + * find_active_uprobe() which takes mmap_lock for reading. + * Thus this install_breakpoint() can not make + * is_trap_at_addr() true right after find_uprobe() + * returns NULL in find_active_uprobe(). + */ mmap_write_lock(mm); vma = find_vma(mm, info->vaddr); if (!vma || !valid_vma(vma, is_register) ||
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