[PATCH v4 2/2] x86: Update _dl_tlsdesc_dynamic to preserve caller-saved registers

[H.J. Lu]

On Fri, Feb 16, 2024 at 4:47 AM Adhemerval Zanella Netto
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 16/02/24 09:37, H.J. Lu wrote:
> > On Fri, Feb 16, 2024 at 4:20 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >>
> >> On Fri, Feb 16, 2024 at 4:18 AM Florian Weimer <fweimer@redhat.com> wrote:
> >>>
> >>> * H. J. Lu:
> >>>
> >>>> On Thu, Feb 15, 2024 at 10:23 PM Florian Weimer <fweimer@redhat.com> wrote:
> >>>>>
> >>>>> * Adhemerval Zanella Netto:
> >>>>>
> >>>>>>> diff --git a/elf/malloc-for-test.map b/elf/malloc-for-test.map
> >>>>>>> new file mode 100644
> >>>>>>> index 0000000000..8437cf4346
> >>>>>>> --- /dev/null
> >>>>>>> +++ b/elf/malloc-for-test.map
> >>>>>>> @@ -0,0 +1,6 @@
> >>>>>>> +GLIBC_2.0 {
> >>>>>>
> >>>>>> You need to use the correct version to override the malloc:
> >>>>>>
> >>>>>> $ gdb --args tst-gnu2-tls2 --direct
> >>>>>> [...]
> >>>>>> (gdb) b apply_tls
> >>>>>> (gdb) r
> >>>>>> Thread 2 "tst-gnu2-tls2" hit Breakpoint 1, apply_tls (p=0x7ffff7bfee80) at tst-gnu2-tls2mod1.c:25
> >>>>>> 25 {
> >>>>>> (gdb) b malloc
> >>>>>> Breakpoint 2 at 0x7ffff7ca8ad0: malloc. (3 locations)
> >>>>>> (gdb) c
> >>>>>> Continuing.
> >>>>>
> >>>>> Why do we need to set a symbol version here?  I think this can be
> >>>>> removed.
> >>>>
> >>>> malloc-for-test.so provides a fake malloc to clobber caller-save
> >>>> registers.  Since malloc in ld.so has a symbol version,
> >>>>
> >>>>   struct r_found_version version;
> >>>>   version.name = symbol_version_string (libc, GLIBC_2_0);
> >>>>   version.hidden = 0;
> >>>>   version.hash = _dl_elf_hash (version.name);
> >>>>   version.filename = NULL;
> >>>>
> >>>>   void *new_calloc = lookup_malloc_symbol (main_map, "calloc", &version);
> >>>>   void *new_free = lookup_malloc_symbol (main_map, "free", &version);
> >>>>   void *new_malloc = lookup_malloc_symbol (main_map, "malloc", &version);
> >>>>   void *new_realloc = lookup_malloc_symbol (main_map, "realloc", &version);
> >>>>
> >>>> malloc in malloc-for-test.so must have the same version.  Otherwise,
> >>>> it won't be used in the test.
> >>>
> >>> I thought that unversioned symbols interpose all versioned symbols.  Has
> >>> this changed in the dynamic linker?
> >>
> >> Only for this case.
> >>
> >
> > I was wrong.  Version isn't needed.  Will fix it.
> >
>
> Indeed, but I think we also improve the register check testing since just clobbering
> on malloc does not really provide much coverage.

Why?  The slow path will always call malloc in malloc-for-test.so.
We can clobber any caller-save registers we need.  Without
malloc-for-test.so, the test doesn't fail for i386 since malloc in libc.so
doesn't use any vector registers.

-- 
H.J.