perf: stop proot's kompat from trapping futex/epoll/fcntl and stripping the vDSO when only the hostname changes

tools/proot/PATCHES.md

@@ -44,3 +44,14 @@ Ported from WinNative (`main`, 53836ca9, "Fix/performance and vac"):
 - `0009-tracee-relatives-sweep.patch` - tracees count their children, so a terminating thread
   with no children or ptracees no longer walks every tracee; the per-stop memory collector is
   emptied instead of freed and reallocated.
+
+Added by DroidDeck:
+
+- `0011-kompat-utsname-only.patch` - `--kernel-release` (the guest's `DroidDeck` hostname) loads
+  kompat, whose filter traps `futex`, `fcntl`, `epoll_pwait`, `pselect6`, `pipe2`, `eventfd2`,
+  `socket` and more, and which strips `AT_SYSINFO_EHDR` on every `execve`, so glibc runs without
+  the vDSO. When the virtual release is not older than the real kernel and the hwcap is left
+  alone, every one of those handlers is a no-op: kompat now traces only `uname`, `sethostname`
+  and `setdomainname` and leaves the auxv as the kernel wrote it. On an SD 8 Gen 2 guest this
+  took a futex ping-pong from 467 to 97 us, `epoll_pwait` from 60 to 0.8 us and `fcntl` from
+  40-107 to 0.4 us, and `clock_gettime` back to the vDSO.

tools/proot/patches/0011-kompat-utsname-only.patch

@@ -0,0 +1,64 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: DroidDeck <droiddeck@users.noreply.github.com>
+Date: Tue, 29 Sep 2026 00:00:00 +0000
+Subject: [PATCH] kompat: only trace the utsname calls when no older kernel is
+ emulated
+
+---
+ src/extension/kompat/kompat.c | 29 +++++++++++++++++++++++++++--
+ 1 file changed, 27 insertions(+), 2 deletions(-)
+
+diff --git a/src/extension/kompat/kompat.c b/src/extension/kompat/kompat.c
+index 5c39942..4cb7dec 100644
+--- a/src/extension/kompat/kompat.c
++++ b/src/extension/kompat/kompat.c
+@@ -985,6 +985,29 @@ static FilteredSysnum filtered_sysnums[] = {
+ 	FILTERED_SYSNUM_END,
+ };
+ 
++/* When the virtual kernel is not older than the actual one and the
++ * hwcap is left alone, the only thing left to emulate is the utsname
++ * itself: every other handler above returns without a change.  */
++static FilteredSysnum utsname_only_sysnums[] = {
++	{ PR_setdomainname,	FILTER_SYSEXIT },
++	{ PR_sethostname,	FILTER_SYSEXIT },
++	{ PR_uname, 		FILTER_SYSEXIT },
++	FILTERED_SYSNUM_END,
++};
++
++/**
++ * Return whether @config only renames the kernel identity (nodename,
++ * domainname, version string) without emulating an older kernel or
++ * another hwcap, in which case futex, fcntl, epoll_pwait, ... don't
++ * have to stop in the tracer and the vDSO can be left in place.
++ */
++static bool utsname_only(const Config *config)
++{
++	return config->actual_release != 0
++		&& config->virtual_release >= config->actual_release
++		&& config->hwcap == (word_t) -1;
++}
++
+ /**
+  * Handler for this @extension.  It is triggered each time an @event
+  * occured.  See ExtensionEvent for the meaning of @data1 and @data2.
+@@ -1007,7 +1030,9 @@ int kompat_callback(Extension *extension, ExtensionEvent event,
+ 		if (status < 0)
+ 			return -1;
+ 
+-		extension->filtered_sysnums = filtered_sysnums;
++		extension->filtered_sysnums = utsname_only(config)
++			? utsname_only_sysnums
++			: filtered_sysnums;
+ 		return 0;
+ 	}
+ 
+@@ -1038,7 +1063,7 @@ int kompat_callback(Extension *extension, ExtensionEvent event,
+ 
+ 		/* Note: this can be done only before PRoot pushes the
+ 		 * load script into tracee's stack.  */
+-		if ((int) result >= 0 && sysnum == PR_execve)
++		if ((int) result >= 0 && sysnum == PR_execve && !utsname_only(config))
+ 			adjust_elf_auxv(tracee, config);
+ 		return 0;
+ 	}