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Is there a way to query the TLS model of a shared library on Linux? (eg using ldd or some other tool).

I am having a trouble with loading too many libraries with the "initial-exec" model and would like to determine for sure which of the third party libs use this model (so I can free up some slots eg by linking statically).

This results in an error: 

 dlopen: cannot load any more object with static TLS

see this question.

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robince
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1 Answers1

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I ran into this error myself, and while investigating it, I came on a mailing list post with this info:

If you link a shared object containing IE-model access relocs, the object will have the DF_STATIC_TLS flag set. By the spec, this means that dlopen might refuse to load it.

Looking at /usr/include/elf.h, we have:

/* Values of `d_un.d_val' in the DT_FLAGS entry.  */
...
#define DF_STATIC_TLS   0x00000010      /* Module uses the static TLS model */

So you need to test if DF_STATIC_TLS is set in the DT_FLAGS entry of the shared library.

To test things, I created a simple piece of code using thread local storage:

static __thread int foo;
void set_foo(int new) {
    foo = new;
}

I then compiled it twice with the two different thread local storage models:

gcc -ftls-model=initial-exec -fPIC -c tls.c  -o tls-initial-exec.o
gcc -shared tls-initial-exec.o -o tls-initial-exec.so

gcc -ftls-model=global-dynamic -fPIC -c tls.c  -o tls-global-dynamic.o
gcc -shared tls-global-dynamic.o -o tls-global-dynamic.so

And sure enough, I can see a difference between the two libraries using readelf:

$ readelf --dynamic tls-initial-exec.so

Dynamic section at offset 0xe00 contains 25 entries:
  Tag        Type                         Name/Value
...
 0x000000000000001e (FLAGS)              STATIC_TLS

The tls-global-dynamic.so version did not have a DT_FLAGS entry, presumably because it didn't have any flags set. So it should be fairly easy to create a script using readelf and grep to find affected libraries.

James Henstridge
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    Thanks very much! Great answer. I also found `readelf -l library | grep TLS` useful. This indicates if there is any thread loca storage at all. It turns out anything with TLS also uses a DTV slot (and so can block later STATIC_TLS objects from loading), but if you load the STATIC_TLS objects first, the non-intial-exec ones use another method and don't take a slot. – robince Apr 13 '14 at 11:47
  • @robince Thank you **so** much for this comment (+1). I could not figure out why loading order mattered – Hammer Sep 02 '20 at 20:12
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    As I recently found out, aarch64 binaries don't have `STATIC_TLS` in `FLAGS` even if static TLS is used. The best I found is to look for relocation entries in `readelf -a -W library | grep R_AARCH64_TLS_TPREL64` output. If there are any, it indicates static TLS binary. The equivalent for x86_64 is `R_X86_64_TPOFF64`. – skazi Jan 25 '22 at 18:13