rseq: Allow registering RSEQ with slice extension

Since glibc cares about the number of syscalls required to initialize a new
thread, allow initializing rseq with slice extension on. This avoids having to
do another prctl().

Requested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260121143207.814193010@infradead.org

diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
index 6afc219d154595c5fe96b582a5efce540b008744..863c4a00a66b1bb25c0cea4b2332db1107b9b6e2 100644 (file)
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -19,7 +19,8 @@ enum rseq_cpu_id_state {
 };
 
 enum rseq_flags {
-       RSEQ_FLAG_UNREGISTER = (1 << 0),
+       RSEQ_FLAG_UNREGISTER                    = (1 << 0),
+       RSEQ_FLAG_SLICE_EXT_DEFAULT_ON          = (1 << 1),
 };
 
 enum rseq_cs_flags_bit {

diff --git a/kernel/rseq.c b/kernel/rseq.c
index 275d701141079fcf0a2938da93f22dda6ad777a6..1c5490a172a87be2071e16016bf78934b8f8c542 100644 (file)
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -424,7 +424,7 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, int, flags, u32
                return 0;
        }
 
-       if (unlikely(flags))
+       if (unlikely(flags & ~(RSEQ_FLAG_SLICE_EXT_DEFAULT_ON)))
                return -EINVAL;
 
        if (current->rseq.usrptr) {
@@ -459,8 +459,12 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, int, flags, u32
        if (!access_ok(rseq, rseq_len))
                return -EFAULT;
 
-       if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION))
+       if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION)) {
                rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
+               if (rseq_slice_extension_enabled() &&
+                   (flags & RSEQ_FLAG_SLICE_EXT_DEFAULT_ON))
+                       rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
+       }
 
        scoped_user_write_access(rseq, efault) {
                /*
@@ -488,6 +492,10 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, int, flags, u32
        current->rseq.len = rseq_len;
        current->rseq.sig = sig;
 
+#ifdef CONFIG_RSEQ_SLICE_EXTENSION
+       current->rseq.slice.state.enabled = !!(rseqfl & RSEQ_CS_FLAG_SLICE_EXT_ENABLED);
+#endif
+
        /*
         * If rseq was previously inactive, and has just been
         * registered, ensure the cpu_id_start and cpu_id fields