On 5/28/24 10:13 PM, Tiwei Bie wrote: > On 5/28/24 7:57 PM, Johannes Berg wrote: >> On Tue, 2024-05-28 at 18:16 +0800, Tiwei Bie wrote: >>> On 5/28/24 4:54 PM, benja...@sipsolutions.net wrote: >>>> From: Benjamin Berg <benjamin.b...@intel.com> >>>> >>>> Newer glibc versions are enabling rseq support by default. This remains >>>> enabled in the cloned child process, potentially causing the host kernel >>>> to write/read memory in the child. >>>> >>>> It appears that this was purely not an issue because the used memory >>>> area happened to be above TASK_SIZE and remains mapped. >>> >>> I also encountered this issue. In my case, with "Force a static link" >>> (CONFIG_STATIC_LINK) enabled, UML will crash immediately every time >>> it starts up. I worked around this by setting the glibc.pthread.rseq >>> tunable via GLIBC_TUNABLES [1] before launching UML. >>> >>> So another easy way to work around this issue without introducing runtime >>> overhead might be to add the GLIBC_TUNABLES=glibc.pthread.rseq=0 environment >>> variable and exec /proc/self/exe in UML on startup. >>> >> >> It's also a bit of a question what to rely on - this would introduce a >> dependency on glibc behaviour, whereas doing the double-clone proposed >> here will work purely because of host kernel behaviour, regardless of >> what part of the system set up rseq, how the tunables work, etc. > > Makes sense. My previous concern was primarily about the runtime overhead, > but after taking a closer look at the patch, I realized that the double-clone > won't happen on the critical path, so there shouldn't be any performance > issues. I also think the double-clone proposal is better. :)
But when combined with this series [1], things might be different.. Double-clone will happen for each new mm context. That's something we might want to avoid. [1] https://patchwork.ozlabs.org/project/linux-um/list/?series=408104 Regards, Tiwei
