RE: [PATCH] gdb: use NT_386_TLS regset to access TLS GDT entries on i386 Linux

["Joos, Christina" <christina.joos@intel.com>]

> -----Original Message-----
> From: Andrew Burgess <aburgess@redhat.com>
> Sent: Dienstag, 29. September 2026 11:38
> To: Simon Marchi <simon.marchi@efficios.com>; gdb-patches@sourceware.org
> Cc: Simon Marchi <simon.marchi@efficios.com>
> Subject: Re: [PATCH] gdb: use NT_386_TLS regset to access TLS GDT entries on
> i386 Linux
> 
> Simon Marchi <simon.marchi@efficios.com> writes:
> 
> > Bug 34678 reports that a 32-bit GDB running on an x86-64 kernel can't
> > make inferior function calls:
> >
> >     (gdb) p f()
> >     Couldn't get TLS area data: Invalid argument.
> >
> > This happens because GDB fails to read the special registers holding
> > the TLS GDT entries, added in commit 91eee81d2353 ("gdb: include
> > NT_I386_TLS note in generated core files", 2025-11-20).
> >
> > The failure isn't specifically related to inferior function calls, but
> > it is most visible there when GDB attempts to save all registers prior
> > to a function call.
> >
> > These "registers" are read using
> >
> >     ptrace (PTRACE_GET_THREAD_AREA, pid, addr, data)
> >
> > where `addr` is not an address, but the index of a GDT entry to read.
> > The valid indices depend on the arch of the kernel.  For an i386
> > kernel, the valid range is [6, 8], while for an x86-64 kernel, the
> > valid range is [12, 14].
> >
> > As commit 91eee81d2353 properly noted, the indices really depend on
> > the kernel, not on how GDB or the inferior program were built.  On an
> > x86-64 kernel, even when GDB and/or the inferior are 32-bit programs,
> > we need to query the x86-64 indices:
> >
> >     /* This constant defines the first GDT (Global Descriptor Table) entry
> >        that the kernel allocates for holding TLS descriptors.  There are three
> >        entries, starting at this index which can be accessed using the
> >        PTRACE_GET_THREAD_AREA and PTRACE_SET_THREAD_AREA ptrace
> calls.  This
> >        constant is only valid for true i386 kernels.  For amd64 kernels
> >        running in 32-bit mode (i.e. executables compiled -m32) there is a
> >        different constant, see nat/amd64-linux.h.  */
> >
> > However, the implementation isn't quite right, since it bases the
> > decision on whether GDB itself is a 32-bit or 64-bit program.  So we
> > get it wrong when GDB is a 32-bit program, debugging a 32-bit program,
> > on a 64-bit kernel.  GDB queries the i386 indices, which gets an
> > EINVAL reply, because it should have used the x86-64 indices.
> >
> > Also, according to Claude (I couldn't test since I don't have a
> > machine with x32 userland), PTRACE_GET_THREAD_AREA and
> > PTRACE_SET_THREAD_AREA are not supported for x32 tracers: the kernel's
> > x32_arch_ptrace doesn't handle them, so they would fail with EIO.  An
> > x32 GDB debugging an i386 program therefore couldn't read the TLS registers
> either.
> >
> > Fix both problems by using the NT_386_TLS regset instead, with
> > PTRACE_GETREGSET and PTRACE_SETREGSET.  This regset contains the three
> > TLS GDT entries, so accessing it doesn't require knowing their indices.
> > When reading, the kernel fills the entry_number field of each entry
> > with the right index.  When writing, it ignores the entry_number fields.
> > This is the same data as the NT_386_TLS core file note, which these
> > registers are used to produce.
> >
> > This also makes it possible to read the three entries with a single
> > ptrace call, instead of three.
> >
> > Remove the i386_initial_tls_gdt constants, which are now unused, along
> > with nat/amd64-linux.h, which only contained one of them.
> >
> > Tested by running gdb.arch/i386-tls-regs.exp on all these
> > configurations:
> >
> >  - x86-64 kernel, 64-bit GDB, native
> >  - x86-64 kernel, 64-bit GDB, gdbserver
> >  - x86-64 kernel, 32-bit GDB, native
> >  - x86-64 kernel, 32-bit GDB, gdbserver
> >  - i386 kernel, 32-bit GDB, native
> >  - i386 kernel, 32-bit GDB, gdbserver
> >
> > This test would previously fail in the "x86-64 kernel, 32-bit GDB"
> > configs.
> >
> > This is a regression in GDB 18, so this patch would need to be
> > cherry-picked to the gdb-18-branch.
> 
> Thanks for fixing this, and for the well written commit message.
> 
> Just today someone emailed me off-list reporting this problem:
> 
> 
> https://inbox.sourceware.org/binutils/CAG_eJLe_vuR93fE6H4hENPh3=R2XE72
> T2tL1ZaTnY93zW2J5-Q@mail.gmail.com
> 
> which did make it to the gdb-help list via a CC on a follow up post, but
> unfortunately never made it into a release blocking bug.  The issue here is that
> PTRACE_GET_THREAD_AREA and PTRACE_SET_THREAD_AREA don't exist for
> older glibc, they were added in glibc 2.27 I believe, which is after the glibc 2.23
> the bug was reported against.
> 
> The NT_386_TLS that this patch uses instead was added in glibc 2.10, so this
> patch should fix that build issue problem on older glibc too.
> 
> For both master and gdb-18-branch:
> 
> Approved-By: Andrew Burgess <aburgess@redhat.com>
> 
> Thanks,
> Andrew

I plan to look at this in a few hours, but since Andrew has already approved it, it's of course fine with me if you go ahead.
This is probably a bit urgent.

Since x32 is mentioned here, just FYI, in case you haven't seen it:
https://www.phoronix.com/news/Linux-x32-ABI-2026

Christina

 

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