Messages in this thread | | | Date | Thu, 15 Mar 2018 16:48:01 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | uprobes misses breakpoint insertion into VM_WRITE mappings |
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Hi,
Erica has been working on extending test-cases for uprobes, and found something unexpected:
Since commit e40cfce626a5 "uprobes: Restrict valid_vma(false) to skip VM_SHARED vmas" uprobes does not insert breakpoints into mappings mprotect'd as writeable.
This issue can be reproduced by compiling a library without PIC (not using GOT), and then concurrently:
A) Load the library (dynamic loader mprotect the code as writeable to do the relocations, and then mprotect as executable),
B) Enable a uprobe through perf.
(it is a race window between the two mprotect syscalls)
It appears that the following restriction in valid_vma() is responsible for this behavior:
if (is_register) flags |= VM_WRITE;
I don't figure a clear explanation for this flag based on the function comment nor the commit changelog. Any idea on whether this is really needed ?
Note that on uprobes unregister, it allows removing a breakpoint event on a writeable mapping, so there is clearly a discrepancy between the level of paranoia associated with registration and unregistration.
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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