Message ID | 20241025033431.36274-1-kevinb@redhat.com |
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Series |
GDB-internal TLS support for Linux targets
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Message
Kevin Buettner
Oct. 25, 2024, 2:26 a.m. UTC
This series of commits adds internal TLS lookup support to GDB for the following Linux target architectures: x86_64, aarch64, ppc64, s390x, and riscv. When available, libthread_db support for TLS lookup is still preferred/used since it should be more accurate. This means that existing TLS support will still work as it did before - this new TLS support will only be used when libthread_db TLS support is not available. (Though it is possible to force internal TLS support to be used via a new maintenance command). Three of the commits in this series provide knowledge about how to translate link map addresses to module ids and how to traverse various TLS data structures. The latter problem is broken into two parts, one which applies to all Linux architectures, and a second which adds architecture specific knowledge about TLS data structures. Translating link map addresses to module ids is tricky. In theory, the module id is available in the link map data structure, but it's not part of the ABI. I ended up implementing two mechanisms for doing this mapping, one for MUSL, and one for GLIBC. For both of these, I think the method that I used is less fragile than attempting to use an offset to the module id field for current versions of these libraries. Traversing TLS data structures starts with obtaining the value of the thread register (or registers for S390X), then finding the field containing the DTV (dynamic thread vector) address within the TCB (thread control block), then using the module id as an index into the DTV in order to obtain the TLS block. For some architectures, the MUSL C library requires that a final adjustment be made to obtain the actual address of the TLS block. This patch set also shows how internal TLS support might be added for i386, however, due to problems with accessing the gsbase register, it doesn't work, so the commit which adds this potential support is then immediately deleted in the next commit. The point of doing this is to make it available in our git repo to anyone who wishes to work on i386 support. IMO, it's not worth doing without also doing corresponding ptrace work in the kernel. I think this would have been worth doing back in the i386 heyday, but is not worth doing now. That said, should anyone wish to look into it, the commit showing how to do it will be in our repo as well as on the mailing list. The details for traversing the TLS data structures differ not only between architectures, but also depends upon the C library with which the executable being debugged has been linked. The internal TLS support in this series is known to work with GLIBC versions 2.27 thru 2.40.9000 and MUSL versions 1.1.24, 1.2.3 and 1.2.5. For MUSL, the support provided by this series provides new debugging functionality that didn't exist before - it will now be possible to examine TLS variables in programs linked against MUSL. (It didn't work before due to MUSL not implementing the libthread_db library.) I've done regression testing on recent Fedora versions for all five architectures. Bugs were found and fixed during that testing. Once that was done, I did even more testing, using a limited number of tests. These include the new tests that I've added, plus those tests with which regression testing identified some problems. The list is: TESTS="gdb.base/tls-dlobj.exp gdb.base/tls-nothreads.exp \ gdb.base/tls-multiobj.exp gdb.threads/tls.exp \ gdb.server/no-thread-db.exp" I tested using targets: unix, native-gdbserver, native-extended-gdbserver, and, for x86_64 targets, I also tested with 32-bit variants: unix/-m32, native-gdbserver/-m32, and native-extended-gdbserver/-m32 I also tested with no CC_FOR_TARGET (which defaults to gcc), CC_FOR_TARGET=musl-gcc, and CC_FOR_TARGET=clang. On Fedora, using CC_FOR_TARGET=musl-gcc causes the program and libraries to be compiled with gcc, but linked against the MUSL C library. I didn't use this option on non-Fedora machines, though my Void linux testing tested using the MUSL library since that's what's installed in that test environment. I also ran additional tests using check-read1 for combos with no CC_FOR_TARGET. Using all sensible combinations of the above, I tested on 13 machines / 5 architectures: x86_64 / Fedora 28 / glibc-2.27 x86_64 / Fedora 34 / glibc-2.33 / musl-libc-1.2.3 x86_64 / Fedora 35 / glibc-2.34 / musl-libc-1.2.3 x86_64 / Fedora 40 / glibc-2.39 / musl-libc-1.2.5 x86_64 / Fedora 41 / glibc-2.40 / musl-libc-1.2.5 x86_64 / rawhide (fc42) / glibc-2.40.9000 / musl-libc-1.2.5 x86_64 / OpenSuse Leap 15.5 / glibc-2.31 / no musl x86_64 / Ubuntu 22.04 / glibc-2.35 / no musl x86_64 / void - 2024-03-14 / no glibc / musl 1.1.24 aarch64 / Fedora 40 / glibc-2.39 / musl-libc-1.2.5 riscv / Fedora 40 / glibc-2.39 / musl-libc-1.2.5 ppc64le / Fedora 41 / glibc-2.40 / musl-libc-1.2.5 s390x / Fedora 40 / glibc-2.39 / musl-libc-1.2.5 The point of testing old Fedora releases is to be able to test older glibc versions. In particular glibc-2.33 and earlier had pthread functionality split into libpthread.so while glibc-2.34 and later place it into libc proper. All of the testing went well except on riscv and s390x with CC_FOR_TARGET=clang. That's six test runs total, and they each show 799 FAILs. The test results show that riscv mostly prints the wrong answer and that s390x shows output like "Cannot access memory at address 0x3fff8d494e8". But this happens regardless of whether internal TLS support or libthread_db support is used. I think it's likely that it's a clang bug of which I can do nothing about (aside from filing a bug report). The v2 series fixed some problems in the gdb.base/tls-dlobj.exp test found by the Linaro regression tester, tweaked a comment in aarch64-linux-tdep.c, included a discussion of what TLS is in the documentation patch, and renamed 'set force-internal-tls-address-lookup' to be a maintenance command. Thanks to Luis and Eli for their feedback on the v1 series. Thanks, too, to Linaro for regression tester feedback. This v3 series makes corrections to the documentation, as requested by Eli. Kevin Buettner (11): Don't attempt to find TLS address when target has no registers Allow TLS access to work in gdb.server/no-thread-db.exp Track and fetch TLS module ids for MUSL and GLIBC Implement internal TLS address lookup for Linux targets Internal TLS support for aarch64, x86_64, riscv, ppc64, and s390x Internal, but disabled, TLS support for i386 Delete disabled i386 internal TLS support New test - gdb.base/tls-nothreads.exp New test - gdb.base/tls-multiobj.exp New test - gdb.base/tls-dlobj.exp Add TLS NEWS entry and document 'set force-internal-tls-address-lookup' command gdb/NEWS | 20 ++ gdb/aarch64-linux-tdep.c | 55 ++++ gdb/amd64-linux-tdep.c | 37 +++ gdb/doc/gdb.texinfo | 50 +++ gdb/findvar.c | 7 +- gdb/linux-tdep.c | 211 ++++++++++++ gdb/linux-tdep.h | 36 +++ gdb/minsyms.c | 8 +- gdb/ppc-linux-tdep.c | 62 ++++ gdb/riscv-linux-tdep.c | 78 +++++ gdb/s390-linux-tdep.c | 43 +++ gdb/solib-svr4.c | 203 +++++++++++- gdb/solib-svr4.h | 12 + gdb/testsuite/gdb.base/tls-common.exp.tcl | 50 +++ gdb/testsuite/gdb.base/tls-dlobj-lib.c | 87 +++++ gdb/testsuite/gdb.base/tls-dlobj.c | 311 ++++++++++++++++++ gdb/testsuite/gdb.base/tls-dlobj.exp | 378 ++++++++++++++++++++++ gdb/testsuite/gdb.base/tls-multiobj.c | 89 +++++ gdb/testsuite/gdb.base/tls-multiobj.exp | 230 +++++++++++++ gdb/testsuite/gdb.base/tls-multiobj1.c | 26 ++ gdb/testsuite/gdb.base/tls-multiobj2.c | 26 ++ gdb/testsuite/gdb.base/tls-multiobj3.c | 26 ++ gdb/testsuite/gdb.base/tls-nothreads.c | 57 ++++ gdb/testsuite/gdb.base/tls-nothreads.exp | 248 ++++++++++++++ gdb/testsuite/gdb.server/no-thread-db.exp | 4 +- 25 files changed, 2347 insertions(+), 7 deletions(-) create mode 100644 gdb/testsuite/gdb.base/tls-common.exp.tcl create mode 100644 gdb/testsuite/gdb.base/tls-dlobj-lib.c create mode 100644 gdb/testsuite/gdb.base/tls-dlobj.c create mode 100644 gdb/testsuite/gdb.base/tls-dlobj.exp create mode 100644 gdb/testsuite/gdb.base/tls-multiobj.c create mode 100644 gdb/testsuite/gdb.base/tls-multiobj.exp create mode 100644 gdb/testsuite/gdb.base/tls-multiobj1.c create mode 100644 gdb/testsuite/gdb.base/tls-multiobj2.c create mode 100644 gdb/testsuite/gdb.base/tls-multiobj3.c create mode 100644 gdb/testsuite/gdb.base/tls-nothreads.c create mode 100644 gdb/testsuite/gdb.base/tls-nothreads.exp