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Subject: Re: linux uml segfault
From: Johannes Berg <johannes@sipsolutions.net>
Date: Fri, 05 Mar 2021 20:54:11 +0100
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To: rrs@debian.org, Anton Ivanov <anton.ivanov@kot-begemot.co.uk>, Christopher Obbard <chris.obbard@collabora.com>, "linux-um@lists.infradead.org" <linux-um@lists.infradead.org>
Cc: 983379@bugs.debian.org

On Wed, 2021-03-03 at 23:40 +0100, Johannes Berg wrote:

> Now libcom_err.so.2 is trying to call sem_init(), and that gets ... tada
> ... Linux's sem_init() instead of libpthread's.
> 
> And then the crash.

FWIW, I can trivially reproduce this by simply force-loading
libcom_err.so:


diff --git a/arch/um/Makefile b/arch/um/Makefile
index 1cea46ff9bb7..a16b411154fb 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -134,7 +134,7 @@ LINK_WRAPS = -Wl,--wrap,malloc -Wl,--wrap,free -Wl,--wrap,calloc
 LD_FLAGS_CMDLINE = $(foreach opt,$(KBUILD_LDFLAGS),-Wl,$(opt))
 
 # Used by link-vmlinux.sh which has special support for um link
-export CFLAGS_vmlinux := $(LINK-y) $(LINK_WRAPS) $(LD_FLAGS_CMDLINE)
+export CFLAGS_vmlinux := $(LINK-y) $(LINK_WRAPS) $(LD_FLAGS_CMDLINE) -ldl
 
 # When cleaning we don't include .config, so we don't include
 # TT or skas makefiles and don't clean skas_ptregs.h.
diff --git a/arch/um/os-Linux/main.c b/arch/um/os-Linux/main.c
index c8a42ecbd7a2..873dc4c40cb7 100644
--- a/arch/um/os-Linux/main.c
+++ b/arch/um/os-Linux/main.c
@@ -16,6 +16,7 @@
 #include <kern_util.h>
 #include <os.h>
 #include <um_malloc.h>
+#include <dlfcn.h>
 
 #define PGD_BOUND (4 * 1024 * 1024)
 #define STACKSIZE (8 * 1024 * 1024)
@@ -115,6 +116,8 @@ int __init main(int argc, char **argv, char **envp)
 
 	setsid();
 
+dlopen("/usr/lib64/libcom_err.so.2", RTLD_NOW|RTLD_GLOBAL);
+
 	new_argv = malloc((argc + 1) * sizeof(char *));
 	if (new_argv == NULL) {
 		perror("Mallocing argv");


johannes


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