----- On Jul 14, 2020, at 10:34 PM, Chris Kennelly ckennelly@xxxxxxxxxx wrote: > On Tue, Jul 14, 2020 at 2:33 PM Peter Oskolkov <posk@xxxxxxxxxx> wrote: >> >> On Tue, Jul 14, 2020 at 10:43 AM Mathieu Desnoyers >> <mathieu.desnoyers@xxxxxxxxxxxx> wrote: >> > >> > ----- On Jul 14, 2020, at 1:24 PM, Peter Oskolkov posk@xxxxxxx wrote: >> > >> > > At Google, we actually extended struct rseq (I will post the patches >> > > here once they are fully deployed and we have specific >> > > benefits/improvements to report). We did this by adding several fields >> > > below __u32 flags (the last field currently), and correspondingly >> > > increasing rseq_len in rseq() syscall. If the kernel does not know of >> > > this extension, it will return -EINVAL due to an unexpected rseq_len; >> > > then the application can either fall-back to the standard/upstream >> > > rseq, or bail. If the kernel does know of this extension, it accepts >> > > it. If the application passes the old rseq_len (32), the kernel knows >> > > that this is an old application and treats it as such. >> > > >> > > I looked through the archives, but I did not find specifically why the >> > > pretty standard approach described above is considered inferior to the >> > > one taken in this patch (freeze rseq_len at 32, add additional length >> > > fields to struct rseq). Can these be summarized? >> > >> > I think you don't face the issues I'm facing with libc rseq integration >> > because you control the entire user-space software ecosystem at Google. >> > >> > The main issue we face is that the library responsible for registering >> > rseq (either glibc 2.32+, an early-adopter librseq library, or the >> > application) may very well not be the same library defining the __rseq_abi >> > symbol used in the global symbol table. Interposition with ld preload or >> > by defining the __rseq_abi in the program's executable are good examples >> > of this kind of scenario, and those use-cases are supported. > > Does this work if/when we run out of bytes in the current sizeof(__rseq_abi)? Only if all libraries/programs involved (including glibc) expect that the size of the __rseq_abi can be the smallest possible subset, and only consider it to be "extended" if specific information in the ABI tells them it is the case. > > Which library provides the TLS symbol (and N bytes of storage) seems > sensitive to the choices the linker makes for us, once the symbol > sizes diverge. AFAIU, a symbol defined in the main executable will have precedence over a preloaded library, which has precedence over shared library dependencies, e.g. glibc. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
