Date: Mon, 6 Sep 2021 07:31:39 -0700 From: "H.J. Lu" <hjl.tools@...il.com> To: Florian Weimer <fweimer@...hat.com> Cc: GNU C Library <libc-alpha@...rceware.org>, GDB <gdb@...rceware.org>, libc-coord@...ts.openwall.com, Daniel Walker <danielwa@...co.com> Subject: Re: [PATCH v6 2/2] Extend struct r_debug to support multiple namespaces On Mon, Sep 6, 2021 at 7:24 AM Florian Weimer <fweimer@...hat.com> wrote: > > * H. J. Lu: > > > On Mon, Sep 6, 2021 at 2:39 AM Florian Weimer <fweimer@...hat.com> wrote: > >> > >> * H. J. Lu: > >> > >> > +* The r_version update in the debugger interface makes the glibc binary > >> > + incompatible with GDB binaries built without the following commits: > >> > + > >> > + c0154a4a21a gdb: Don't assume r_ldsomap when r_version > 1 on Linux > >> > + 4eb629d50d4 gdbserver: Check r_version < 1 for Linux debugger interface > >> > >> Does this incompatibility happen even if audit modules and dlmopen are > >> not used? > > > > Yes. > > Why? Can't we keep r_version at 1 in this case? r_version is checked in GDB whenever DT_DEBUG is used to access the rtld debug interface, independent of audit modules and dlmopen. We can bump r_version only if a non-default namespace is used. > >> This puts the assember output from the compiler through the > >> preprocessor. That seems to be brittle. I think you would have to > >> preprocess the manually written fragment separately. > >> > >> However, I think we are overdesigning things here. The following in > >> dl-debug-symbols-gen.c should work (and the file should have a different > >> name then): > >> > >> /* Alias _r_debug to a prefix of _r_debug_extended. */ > >> asm (".set _r_debug, _r_debug_extended\n\t" > >> ".type _r_debug, %object\n\t" > >> ".symver _r_debug_extended, _r_debug@@" FIRST_VERSION_ld__r_debug_STRING); > >> #if __WORDSIZE == 64 > >> _Static_assert (sizeof (struct r_debug) == 40, "sizeof (struct r_debug)"); > >> asm (".size _r_debug, 40"); > >> #else > >> _Static_assert (sizeof (struct r_debug) == 20, "sizeof (struct r_debug)"); > >> asm (".size _r_debug, 20"); > >> #endif > >> > >> It's not exactly pretty, but at least it's obvious what is going on. > >> (Extended asm with input operands is not supported outside of functions.) > > > > This was the first thing I tried and it didn't work: > > > > [hjl@...-cfl-2 tmp]$ cat foo.s > > .set _r_debug, _r_debug_extended > > .globl _r_debug > > .type _r_debug, %object > > .size _r_debug, 40 > > .data > > .type _r_debug_extended, %object > > .size _r_debug_extended, 48 > > .globl _r_debug_extended > > _r_debug_extended: > > .zero 48 > > [hjl@...-cfl-2 tmp]$ gcc -c foo.s > > [hjl@...-cfl-2 tmp]$ readelf -sW foo.o | grep _r_debug > > 1: 0000000000000000 48 OBJECT GLOBAL DEFAULT 2 _r_debug > > 2: 0000000000000000 48 OBJECT GLOBAL DEFAULT 2 _r_debug_extended > > [hjl@...-cfl-2 tmp]$ > > Huh. Does this mean this depends on the symbol definition order in the > assembler file? > > I really hate the post-processing of compiler output. This isn't GHC. 8-> > > Can we write a portable assembler file instead? > > Nick Clifton has written down some guidelines: > > Tips for writing portable assembler with GNU Assembler (GAS) > <https://developers.redhat.com/blog/2021/02/26/tips-for-writing-portable-assembler-with-gnu-assembler-gas> > > There's no initializer, so all we need to know is size and alignment. Yes, I can do that. > >> Is this necessary? It makes concurrent access to the list harder and > > > > When _dl_close_worker is called, it holds GL(dl_load_lock). Why does > > this change make concurrent access harder? > > Something else might want to read the list directly, by starting with > DT_DEBUG. I can add _dl_debug_get and use _dl_debug_initialize only for initialization. Will it work? -- H.J.
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