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Re: [RFC PATCH] glibc: Perform rseq(2) registration at nptl init and thread creation


On 19/09/18 15:44, Mathieu Desnoyers wrote:
Things to consider:

- Move __rseq_refcount to an extra field at the end of __rseq_abi to
   eliminate one symbol. This would require to wrap struct rseq into
   e.g. struct rseq_lib or such, e.g.:

struct rseq_lib {
   struct rseq kabi;
   int refcount;
};

All libraries/programs which try to register rseq (glibc, early-adopter
applications, early-adopter libraries) should use the rseq refcount.
It becomes part of the ABI within a user-space process, but it's not
part of the ABI shared with the kernel per se.

- Restructure how this code is organized so glibc keeps building on
   non-Linux targets.

- We do not need an atomic increment/decrement for the refcount per
   se. Just being atomic with respect to the current thread (and nested
   signals) would be enough. What is the proper API to use there ?
   Should we expose struct rseq_lib in a public glibc header ? Should
   we create a rseq(3) man page ?

- Revisit use of "weak" symbol for __rseq_abi in glibc. Perhaps we
   want a non-weak symbol there ? (and let all other early user
   libraries use weak)


i don't think there is precedent for exposing tls symbol in glibc
(e.g. errno is exposed via __errno_location function) so there
might be issues with this (but i don't have immediate concerns).

diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
index fe75d04113..20ee197d94 100644
--- a/nptl/pthread_create.c
+++ b/nptl/pthread_create.c
@@ -52,6 +52,13 @@ static struct pthread *__nptl_last_event __attribute_used__;
  /* Number of threads running.  */
  unsigned int __nptl_nthreads = 1;
+__attribute__((weak, tls_model("initial-exec"))) __thread
+volatile struct rseq __rseq_abi = {
+	.cpu_id = RSEQ_CPU_ID_UNINITIALIZED,
+};
+
+__attribute__((weak, tls_model("initial-exec"))) __thread
+volatile int __rseq_refcount;

note that libpthread.so is built with -ftls-model=initial-exec

(and if it wasn't then you'd want to put the attribute on the
declaration in the internal header file, not on the definition,
so the actual tls accesses generate the right code)


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