runtime: debug.decoratemappings don't work as expected

[patryk4815]

Go version

go1.25+ and master

Output of go env in your module/workspace:

.

What did you do?

I copied program from: 52eaed6

package main
import (
        "fmt"
        "log"
        "os"
)

/*
#include <stdlib.h>

void f(void)
{
  (void)malloc(1024*1024*1024);
}
*/
import "C"

func main() {
        C.f()

        data, err := os.ReadFile("/proc/self/maps")
        if err != nil {
                log.Fatal(err)
        }
        fmt.Println(string(data))
}

What did you see happen?

When I run this program on go1.25
I see only single named memory mapping [anon: Go: immortal metadata]

patryk@patryk:~$ GLIBC_TUNABLES=glibc.mem.decorate_maps=1 go run a.go
# command-line-arguments
./a.go: In function \u2018f\u2019:
./a.go:13:9: warning: ignoring return value of \u2018malloc\u2019 declared with attribute \u2018warn_unused_result\u2019 [-Wunused-result]
   13 |   (void)malloc(1024*1024*1024);
      |         ^~~~~~~~~~~~~~~~~~~~~~
00400000-00577000 r-xp 00000000 fd:02 16384750                           /home/patryk/.cache/go-build/bb/bbe5287252af91f0dcae2fc8323a2bafe87257c1b844db57cd3c7711fb5abed7-d/a
0058f000-00590000 r--p 0017f000 fd:02 16384750                           /home/patryk/.cache/go-build/bb/bbe5287252af91f0dcae2fc8323a2bafe87257c1b844db57cd3c7711fb5abed7-d/a
00590000-0059b000 rw-p 00180000 fd:02 16384750                           /home/patryk/.cache/go-build/bb/bbe5287252af91f0dcae2fc8323a2bafe87257c1b844db57cd3c7711fb5abed7-d/a
0059b000-005d7000 rw-p 00000000 00:00 0 
00ece000-00eef000 rw-p 00000000 00:00 0                                  [heap]
4000000000-4000400000 rw-p 00000000 00:00 0 
4000400000-4004000000 ---p 00000000 00:00 0 
f71a54000000-f71a54021000 rw-p 00000000 00:00 0                          [anon: glibc: malloc arena]
f71a54021000-f71a58000000 ---p 00000000 00:00 0 
f71a58000000-f71a58021000 rw-p 00000000 00:00 0                          [anon: glibc: malloc arena]
f71a58021000-f71a5c000000 ---p 00000000 00:00 0 
f71a5c000000-f71a5c021000 rw-p 00000000 00:00 0                          [anon: glibc: malloc arena]
f71a5c021000-f71a60000000 ---p 00000000 00:00 0 
f71a60000000-f71a60021000 rw-p 00000000 00:00 0                          [anon: glibc: malloc arena]
f71a60021000-f71a64000000 ---p 00000000 00:00 0 
f71a64000000-f71a64021000 rw-p 00000000 00:00 0                          [anon: glibc: malloc arena]
f71a64021000-f71a68000000 ---p 00000000 00:00 0 
f71a68230000-f71a68240000 ---p 00000000 00:00 0 
f71a68240000-f71a68a40000 rw-p 00000000 00:00 0                          [anon: glibc: pthread stack: 608584]
f71a68a40000-f71a68a50000 ---p 00000000 00:00 0 
f71a68a50000-f71a69250000 rw-p 00000000 00:00 0                          [anon: glibc: pthread stack: 608583]
f71a69250000-f71a69290000 rw-p 00000000 00:00 0                          [anon: Go: immortal metadata]
f71a69290000-f71a692a0000 ---p 00000000 00:00 0 
f71a692a0000-f71a69aa0000 rw-p 00000000 00:00 0                          [anon: glibc: pthread stack: 608582]
f71a69aa0000-f71a69ab0000 ---p 00000000 00:00 0 
f71a69ab0000-f71a6a2b0000 rw-p 00000000 00:00 0                          [anon: glibc: pthread stack: 608581]
f71a6a2b0000-f71a6a2f0000 rw-p 00000000 00:00 0                          [anon: Go: immortal metadata]
f71a6a2f0000-f71a6a300000 ---p 00000000 00:00 0 
f71a6a300000-f71a6ab00000 rw-p 00000000 00:00 0                          [anon: glibc: pthread stack: 608580]
f71a6ab00000-f71a6cc00000 rw-p 00000000 00:00 0 
f71a6cc00000-f71a6cc80000 ---p 00000000 00:00 0 
f71a6cc80000-f71a6cc81000 rw-p 00000000 00:00 0 
f71a6cc81000-f71a8cc80000 ---p 00000000 00:00 0 
f71a8cc80000-f71a8cc81000 rw-p 00000000 00:00 0 
f71a8cc81000-f71aacc10000 ---p 00000000 00:00 0 
f71aacc10000-f71aacc11000 rw-p 00000000 00:00 0 
f71aacc11000-f71ab0c02000 ---p 00000000 00:00 0 
f71ab0c02000-f71ab0c03000 rw-p 00000000 00:00 0 
f71ab0c03000-f71ab1400000 ---p 00000000 00:00 0 
f71ab142e000-f71ab1461000 rw-p 00000000 00:00 0 
f71ab1461000-f71ab1560000 ---p 00000000 00:00 0 
f71ab1560000-f71ab15c0000 rw-p 00000000 00:00 0 
f71ab15c0000-f71ab176a000 r-xp 00000000 fd:02 18747670                   /usr/lib/aarch64-linux-gnu/libc.so.6
f71ab176a000-f71ab177d000 ---p 001aa000 fd:02 18747670                   /usr/lib/aarch64-linux-gnu/libc.so.6
f71ab177d000-f71ab1780000 r--p 001ad000 fd:02 18747670                   /usr/lib/aarch64-linux-gnu/libc.so.6
f71ab1780000-f71ab1782000 rw-p 001b0000 fd:02 18747670                   /usr/lib/aarch64-linux-gnu/libc.so.6
f71ab1782000-f71ab178f000 rw-p 00000000 00:00 0 
f71ab179e000-f71ab17c7000 r-xp 00000000 fd:02 18747364                   /usr/lib/aarch64-linux-gnu/ld-linux-aarch64.so.1
f71ab17d6000-f71ab17d8000 rw-p 00000000 00:00 0                          [anon: glibc: loader malloc]
f71ab17d8000-f71ab17da000 r--p 00000000 00:00 0                          [vvar]
f71ab17da000-f71ab17dc000 r-xp 00000000 00:00 0                          [vdso]
f71ab17dc000-f71ab17de000 r--p 0002e000 fd:02 18747364                   /usr/lib/aarch64-linux-gnu/ld-linux-aarch64.so.1
f71ab17de000-f71ab17df000 rw-p 00030000 fd:02 18747364                   /usr/lib/aarch64-linux-gnu/ld-linux-aarch64.so.1
f71ab17df000-f71ab17e0000 rw-p 00000000 00:00 0 
ffffe0934000-ffffe0956000 rw-p 00000000 00:00 0                          [stack]

What did you expect to see?

Fix should be backported to go1.25.
I think the main issue is that debug.decoratemappings is initialized with 0, and only later at runtime it changes to 1 - too late to take effect.

After applying my fix:

patryk@patryk:~/goroot/src$ git diff
diff --git a/src/runtime/set_vma_name_linux.go b/src/runtime/set_vma_name_linux.go
index 9b6654f332..d6b1a3c5a7 100644
--- a/src/runtime/set_vma_name_linux.go
+++ b/src/runtime/set_vma_name_linux.go
@@ -16,7 +16,7 @@ var prSetVMAUnsupported atomic.Bool
 
 // setVMAName calls prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME, start, len, name)
 func setVMAName(start unsafe.Pointer, length uintptr, name string) {
-       if debug.decoratemappings == 0 || prSetVMAUnsupported.Load() {
+       if prSetVMAUnsupported.Load() {
                return
        }
 

I see this:

patryk@patryk:~$ GLIBC_TUNABLES=glibc.mem.decorate_maps=1 go run a.go 
# command-line-arguments
./a.go: In function \u2018f\u2019:
./a.go:13:9: warning: ignoring return value of \u2018malloc\u2019 declared with attribute \u2018warn_unused_result\u2019 [-Wunused-result]
   13 |   (void)malloc(1024*1024*1024);
      |         ^~~~~~~~~~~~~~~~~~~~~~
00400000-00574000 r-xp 00000000 00:2d 441968                             /tmp/go-build4081026567/b001/exe/a
0058f000-00590000 r--p 0017f000 00:2d 441968                             /tmp/go-build4081026567/b001/exe/a
00590000-0059b000 rw-p 00180000 00:2d 441968                             /tmp/go-build4081026567/b001/exe/a
0059b000-005d7000 rw-p 00000000 00:00 0 
3f99c000-3f9bd000 rw-p 00000000 00:00 0                                  [heap]
4000000000-4000400000 rw-p 00000000 00:00 0                              [anon: Go: heap]
4000400000-4004000000 ---p 00000000 00:00 0                              [anon: Go: heap reservation]
f856a0000000-f856a0021000 rw-p 00000000 00:00 0                          [anon: glibc: malloc arena]
f856a0021000-f856a4000000 ---p 00000000 00:00 0 
f856a8000000-f856a8021000 rw-p 00000000 00:00 0                          [anon: glibc: malloc arena]
f856a8021000-f856ac000000 ---p 00000000 00:00 0 
f856ac000000-f856ac021000 rw-p 00000000 00:00 0                          [anon: glibc: malloc arena]
f856ac021000-f856b0000000 ---p 00000000 00:00 0 
f856b0000000-f856b0021000 rw-p 00000000 00:00 0                          [anon: glibc: malloc arena]
f856b0021000-f856b4000000 ---p 00000000 00:00 0 
f856b4630000-f856b4640000 ---p 00000000 00:00 0 
f856b4640000-f856b4e40000 rw-p 00000000 00:00 0 
f856b4e40000-f856b4e50000 ---p 00000000 00:00 0 
f856b4e50000-f856b5650000 rw-p 00000000 00:00 0                          [anon: glibc: pthread stack: 608452]
f856b5650000-f856b5690000 rw-p 00000000 00:00 0                          [anon: Go: immortal metadata]
f856b5690000-f856b56a0000 ---p 00000000 00:00 0 
f856b56a0000-f856b5ea0000 rw-p 00000000 00:00 0                          [anon: glibc: pthread stack: 608451]
f856b5ea0000-f856b5eb0000 ---p 00000000 00:00 0 
f856b5eb0000-f856b66b0000 rw-p 00000000 00:00 0                          [anon: glibc: pthread stack: 608450]
f856b66b0000-f856b66f0000 rw-p 00000000 00:00 0                          [anon: Go: immortal metadata]
f856b66f0000-f856b6700000 ---p 00000000 00:00 0 
f856b6700000-f856b6f00000 rw-p 00000000 00:00 0                          [anon: glibc: pthread stack: 608449]
f856b6f00000-f856b7000000 rw-p 00000000 00:00 0                          [anon: Go: page alloc index]
f856b7000000-f856b9000000 rw-p 00000000 00:00 0                          [anon: Go: heap index]
f856b9000000-f856b9080000 ---p 00000000 00:00 0                          [anon: Go: scavenge index]
f856b9080000-f856b9081000 rw-p 00000000 00:00 0                          [anon: Go: scavenge index]
f856b9081000-f856d9000000 ---p 00000000 00:00 0                          [anon: Go: scavenge index]
f856d9000000-f856d9080000 ---p 00000000 00:00 0                          [anon: Go: page summary]
f856d9080000-f856d9081000 rw-p 00000000 00:00 0                          [anon: Go: page alloc]
f856d9081000-f856f9010000 ---p 00000000 00:00 0                          [anon: Go: page summary]
f856f9010000-f856f9011000 rw-p 00000000 00:00 0                          [anon: Go: page alloc]
f856f9011000-f856fd002000 ---p 00000000 00:00 0                          [anon: Go: page summary]
f856fd002000-f856fd003000 rw-p 00000000 00:00 0                          [anon: Go: page alloc]
f856fd003000-f856fd800000 ---p 00000000 00:00 0                          [anon: Go: page summary]
f856fd80e000-f856fd81e000 rw-p 00000000 00:00 0                          [anon: Go: gc bits]
f856fd81e000-f856fd82e000 rw-p 00000000 00:00 0                          [anon: Go: allspans array]
f856fd82e000-f856fd840000 rw-p 00000000 00:00 0                          [anon: Go: immortal metadata]
f856fd840000-f856fd841000 rw-p 00000000 00:00 0                          [anon: Go: page alloc]
f856fd841000-f856fd940000 ---p 00000000 00:00 0                          [anon: Go: page summary]
f856fd940000-f856fd960000 rw-p 00000000 00:00 0                          [anon: Go: page alloc]
f856fd960000-f856fd9a0000 rw-p 00000000 00:00 0                          [anon: Go: immortal metadata]
f856fd9a0000-f856fdb4a000 r-xp 00000000 fd:02 18747670                   /usr/lib/aarch64-linux-gnu/libc.so.6
f856fdb4a000-f856fdb5d000 ---p 001aa000 fd:02 18747670                   /usr/lib/aarch64-linux-gnu/libc.so.6
f856fdb5d000-f856fdb60000 r--p 001ad000 fd:02 18747670                   /usr/lib/aarch64-linux-gnu/libc.so.6
f856fdb60000-f856fdb62000 rw-p 001b0000 fd:02 18747670                   /usr/lib/aarch64-linux-gnu/libc.so.6
f856fdb62000-f856fdb6f000 rw-p 00000000 00:00 0 
f856fdb79000-f856fdba2000 r-xp 00000000 fd:02 18747364                   /usr/lib/aarch64-linux-gnu/ld-linux-aarch64.so.1
f856fdbb1000-f856fdbb3000 rw-p 00000000 00:00 0                          [anon: glibc: loader malloc]
f856fdbb3000-f856fdbb5000 r--p 00000000 00:00 0                          [vvar]
f856fdbb5000-f856fdbb7000 r-xp 00000000 00:00 0                          [vdso]
f856fdbb7000-f856fdbb9000 r--p 0002e000 fd:02 18747364                   /usr/lib/aarch64-linux-gnu/ld-linux-aarch64.so.1
f856fdbb9000-f856fdbba000 rw-p 00030000 fd:02 18747364                   /usr/lib/aarch64-linux-gnu/ld-linux-aarch64.so.1
f856fdbba000-f856fdbbb000 rw-p 00000000 00:00 0 
fffff27de000-fffff27ff000 rw-p 00000000 00:00 0                          [stack]

[gabyhelp]

Related Issues

• runtime: GC & debug.FreeOSMemory calls don't return memory to the OS #45000 (closed)
• runtime: CGo thread stacks stay in physical memory #71150
• cmd/compile: value assigned to global fails to escape #29000 (closed)
• runtime: decorate anonymous memory mappings #71546 (closed)
• debug/pe: fatal error: runtime: out of memory on NewFile #43827
• runtime: ignore map size hint if there's not enough memory? #24308
• reflect: panic from malloc after MakeFunc function returns value that is also stored globally #39541 (closed)
• cmd/compile: runs out of memory allocating 12k + items in a map at init, works as literal map #13554 (closed)
• garbage collection issue in FreeBSD #376 (closed)

Related Code Changes

• runtime: decorate anonymous memory mappings

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[mrkfrmn]

cc @golang/runtime

[mknyszek]

Indeed. We make mappings long before we run any init functions, which is where the default would be set. Even using getGodebugEarly is too late, since at that point we'll have executed mallocinit. Looking at the code, my impression is that it might be safe to call much earlier, since it just constructs a Go string whose backing store is argv.

[gopherbot]

Change https://go.dev/cl/703476 mentions this issue: runtime: initialise debug settings much earlier in startup process