Re: [RFC PATCH v1 00/25] unwind_user: Implement .eh_frame handling

From 159003ef0ef2107ddc8ec8fd8e6ce06198ebbfdc Mon Sep 17 00:00:00 2001
From: Jens Remus <[email protected]>
Date: Wed, 19 Aug 2026 10:15:53 +0200
Subject: [PATCH] TEST: glibc: Register GNU_EH_FRAME PHDRs to the kernel

The kernel does not have direct visibility to the ELF contents of
shared libraries.  In the dynamic linker register and unregister
.eh_frame_hdr sections referenced by GNU_EH_FRAME program headers
via a proposed prctl() interface [1] to the kernel, so it knows
where to find these for stacktracing purposes.

Register .eh_frame_hdr sections during shared object loading and
during setup of VDSO.  Unregister them during unmapping.  Uses
proposed PR_{REGISTER|UNREGISTER}_EH_FRAME prctl.

Based on Josh Poimboeuf's respective GNU_SFRAME test patch.

[1]: [RFC PATCH v1 25/25] unwind_user/eh_frame: Add prctl() interface
     for (un)registering .eh_frame_hdr sections,
     https://lore.kernel.org/all/[email protected]/

[ This patch is for test purposes only. ]

Signed-off-by: Jens Remus <[email protected]>
---
 elf/dl-load.c           | 46 ++++++++++++++++++++++++++++++++++++++++-
 elf/dl-unmap-segments.h | 30 +++++++++++++++++++++++++++
 elf/setup-vdso.h        | 40 +++++++++++++++++++++++++++++++++++
 include/link.h          |  3 +++
 4 files changed, 118 insertions(+), 1 deletion(-)

diff --git a/elf/dl-load.c b/elf/dl-load.c
index 95404adae942..10ef365bc81b 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -29,6 +29,7 @@
 #include <bits/wordsize.h>
 #include <sys/mman.h>
 #include <sys/param.h>
+#include <sys/prctl.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <gnu/lib-names.h>
@@ -65,6 +66,23 @@
 
 #define STRING(x) __STRING (x)
 
+#ifndef _FUTURE_LINUX_EH_FRAME
+#define _FUTURE_LINUX_EH_FRAME
+
+/* From <linux/prctl.h>.  */
+#define PR_REGISTER_EH_FRAME 82
+#define PR_UNREGISTER_EH_FRAME 83
+
+/* From <linux/eh_frame.h>.  */
+struct eh_frame_setup {
+  __u64 eh_frame_hdr_start;
+  __u64 eh_frame_hdr_size;
+  __u64 text_start;
+  __u64 text_size;
+};
+
+#endif /* _FUTURE_LINUX_EH_FRAME  */
+
 
 /* This is the decomposed LD_LIBRARY_PATH search path.  */
 struct r_search_path_struct __rtld_env_path_list attribute_relro;
@@ -1096,6 +1114,11 @@ _dl_map_object_scan_phdrs (struct dl_pt_load_iterator *it,
 	  l->l_relro_addr = ph->p_vaddr;
 	  l->l_relro_size = ph->p_memsz;
 	  break;
+
+	case PT_GNU_EH_FRAME:
+	  l->l_eh_frame_start = ph->p_vaddr;
+	  l->l_eh_frame_size  = ph->p_memsz;
+	  break;
 	}
     }
 
@@ -1384,13 +1407,34 @@ cannot enable executable stack as shared object requires");
   if (l->l_tls_initimage != NULL)
     l->l_tls_initimage = (void*)((uintptr_t)l->l_tls_initimage + l->l_addr);
 
+  /* Adjust the address of the .eh_frame_hdr start address.  */
+  if (l->l_eh_frame_start != 0)
+    l->l_eh_frame_start += l->l_addr;
+
   /* Process program headers again after load segments are mapped in
      case processing requires accessing those segments.  Scan program
      headers backward since PT_GNU_PROPERTY is close to the end of
      program headers.  */
   for (ph = &l->l_phdr[l->l_phnum]; ph != l->l_phdr; --ph)
-    if (ph[-1].p_type == PT_GNU_PROPERTY)
+    switch (ph[-1].p_type)
       {
+      case PT_LOAD:
+	  if (l->l_eh_frame_start != 0
+	      && ph[-1].p_flags & PF_X)
+	    {
+	      ElfW(Addr) text_start = l->l_addr + ph[-1].p_vaddr;
+	      size_t text_size      = ph[-1].p_memsz;
+	      struct eh_frame_setup data = {
+		.eh_frame_hdr_start = l->l_eh_frame_start,
+		.eh_frame_hdr_size  = l->l_eh_frame_size,
+		.text_start         = text_start,
+		.text_size          = text_size,
+	      };
+
+	      __prctl(PR_REGISTER_EH_FRAME, &data, sizeof(data), 0, 0);
+	    }
+	break;
+      case PT_GNU_PROPERTY:
 	_dl_process_pt_gnu_property (l, fd, &ph[-1]);
 	break;
       }
diff --git a/elf/dl-unmap-segments.h b/elf/dl-unmap-segments.h
index c3e46d50ec72..eae0843e85a8 100644
--- a/elf/dl-unmap-segments.h
+++ b/elf/dl-unmap-segments.h
@@ -21,6 +21,24 @@
 
 #include <link.h>
 #include <sys/mman.h>
+#include <sys/prctl.h>
+
+#ifndef _FUTURE_LINUX_EH_FRAME
+#define _FUTURE_LINUX_EH_FRAME
+
+/* From <linux/prctl.h>.  */
+#define PR_REGISTER_EH_FRAME 82
+#define PR_UNREGISTER_EH_FRAME 83
+
+/* From <linux/eh_frame.h>.  */
+struct eh_frame_setup {
+  __u64 eh_frame_hdr_start;
+  __u64 eh_frame_hdr_size;
+  __u64 text_start;
+  __u64 text_size;
+};
+
+#endif /* _FUTURE_LINUX_EH_FRAME  */
 
 /* _dl_map_segments ensures that any whole pages in gaps between segments
    are filled in with PROT_NONE mappings.  So we can just unmap the whole
@@ -29,6 +47,18 @@
 static __always_inline void
 _dl_unmap_segments (struct link_map *l)
 {
+  if (l->l_eh_frame_start != 0)
+    {
+      struct eh_frame_setup data = {
+	.eh_frame_hdr_start = l->l_eh_frame_start,
+	.eh_frame_hdr_size  = 0,
+	.text_start         = 0,
+	.text_size          = 0,
+      };
+
+      __prctl(PR_UNREGISTER_EH_FRAME, &data, sizeof(data), 0, 0);
+    }
+
   __munmap ((void *) l->l_map_start, l->l_map_end - l->l_map_start);
 }
 
diff --git a/elf/setup-vdso.h b/elf/setup-vdso.h
index 0dba7072d53b..83411c1aa396 100644
--- a/elf/setup-vdso.h
+++ b/elf/setup-vdso.h
@@ -16,6 +16,25 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <sys/prctl.h>
+
+#ifndef _FUTURE_LINUX_EH_FRAME
+#define _FUTURE_LINUX_EH_FRAME
+
+/* From <linux/prctl.h>.  */
+#define PR_REGISTER_EH_FRAME 82
+#define PR_UNREGISTER_EH_FRAME 83
+
+/* From <linux/eh_frame.h>.  */
+struct eh_frame_setup {
+  __u64 eh_frame_hdr_start;
+  __u64 eh_frame_hdr_size;
+  __u64 text_start;
+  __u64 text_size;
+};
+
+#endif /* _FUTURE_LINUX_EH_FRAME  */
+
 static inline void __attribute__ ((always_inline))
 setup_vdso (struct link_map *main_map __attribute__ ((unused)),
 	    struct link_map ***first_preload __attribute__ ((unused)))
@@ -56,6 +75,14 @@ setup_vdso (struct link_map *main_map __attribute__ ((unused)),
 	      if (ph->p_vaddr + ph->p_memsz >= l->l_map_end)
 		l->l_map_end = ph->p_vaddr + ph->p_memsz;
 	    }
+	  else if (ph->p_type == PT_GNU_EH_FRAME)
+	    {
+	      if (! l->l_eh_frame_start)
+		{
+		  l->l_eh_frame_start = ph->p_vaddr;
+		  l->l_eh_frame_size  = ph->p_memsz;
+		}
+	    }
 	  else
 	    /* There must be no TLS segment.  */
 	    assert (ph->p_type != PT_TLS);
@@ -78,6 +105,19 @@ setup_vdso (struct link_map *main_map __attribute__ ((unused)),
       l->l_local_scope[0]->r_nlist = 1;
       l->l_local_scope[0]->r_list = &l->l_real;
 
+      if (l->l_eh_frame_start != 0)
+	{
+	  l->l_eh_frame_start += l->l_addr;
+
+	  struct eh_frame_setup data = {
+	    .eh_frame_hdr_start = l->l_eh_frame_start,
+	    .eh_frame_hdr_size  = l->l_eh_frame_size,
+	    .text_start         = l->l_map_start,
+	    .text_size          = l->l_map_end - l->l_map_start,
+	  };
+	  __prctl(PR_REGISTER_EH_FRAME, &data, sizeof(data), 0, 0);
+	}
+
       /* Now that we have the info handy, use the DSO image's soname
 	 so this object can be looked up by name.  */
       {
diff --git a/include/link.h b/include/link.h
index 8f851d2212df..b9ee4674ac69 100644
--- a/include/link.h
+++ b/include/link.h
@@ -345,6 +345,9 @@ struct link_map
     ElfW(Addr) l_relro_addr;
     size_t l_relro_size;
 
+    ElfW(Addr) l_eh_frame_start;
+    size_t l_eh_frame_size;
+
     unsigned long long int l_serial;
   };
 
-- 
2.53.0