linux uml segfault

[Johannes Berg]

On Thu, 2021-03-04 at 14:38 +0900, Hajime Tazaki wrote:
> 
> objcopy (from binutils) can localize symbols (i.e., objcopy -L
> sem_init $orig_file $new_file).

This doesn't seem to be sufficient.

> It also does renaming symbols.  But
> not sure this is the ideal solution.

Even that doesn't seem to actually work/help? I still get libcom_err
trying to call UML's sem_init, even after doing
 objcopy --redefine-sym sem_init=uml_sem_init


> How does UML handle symbol conflicts between userspace code and Linux
> kernel (like this case sem_init) ?  AFAIK, libnl has a same symbol as
> Linux kernel (genlmsg_put) and others can possibly do as well.

I think like I said it just doesn't but since you don't have much
userspace code linked with UML it never really mattered?

We only link a 'linux' binary, after all. How does LKL handle this
though? It should be far more affected?


Despite the objcopy *not* fixing it, this does seem to:

diff --git a/arch/um/kernel/dyn.lds.S b/arch/um/kernel/dyn.lds.S
index dacbfabf66d8..2f2a8ce92f1e 100644
--- a/arch/um/kernel/dyn.lds.S
+++ b/arch/um/kernel/dyn.lds.S
@@ -6,6 +6,12 @@ OUTPUT_ARCH(ELF_ARCH)
 ENTRY(_start)
 jiffies = jiffies_64;
 
+VERSION {
+  {
+    local: *;
+  };
+}
+
 SECTIONS
 {
   PROVIDE (__executable_start = START);
diff --git a/arch/um/kernel/uml.lds.S b/arch/um/kernel/uml.lds.S
index 45d957d7004c..7a8e2b123e29 100644
--- a/arch/um/kernel/uml.lds.S
+++ b/arch/um/kernel/uml.lds.S
@@ -7,6 +7,12 @@ OUTPUT_ARCH(ELF_ARCH)
 ENTRY(_start)
 jiffies = jiffies_64;
 
+VERSION {
+  {
+    local: *;
+  };
+}
+
 SECTIONS
 {
   /* This must contain the right address - not quite the default ELF one.*/

johannes