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Wed, 8 May 2024 22:58:06 +0000 (UTC) (envelope-from jhb@FreeBSD.org) Message-ID: <39488f63-8612-47fd-93a7-022f2ff24a7a@FreeBSD.org> Date: Wed, 8 May 2024 15:58:05 -0700 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv6 7/9] gdb/gdbserver: share some code relating to target description creation Content-Language: en-US To: Andrew Burgess , gdb-patches@sourceware.org Cc: felix.willgerodt@intel.com References: From: John Baldwin In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-4.8 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_MSPIKE_H4,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_PASS,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On 5/8/24 9:46 AM, Andrew Burgess wrote: > This commit is part of a series to share more of the x86 target > description creation code between GDB and gdbserver. > > Unlike previous commits which were mostly refactoring, this commit is > the first that makes a real change, though that change should mostly > be for gdbserver; I've largely adopted the "GDB" way of doing things > for gdbserver, and this fixes a real gdbserver bug. > > On a x86-64 Linux target, running the test: > > gdb.server/connect-with-no-symbol-file.exp > > results in two core files being created. Both of these core files are > from the inferior process, created after gdbserver has detached. > > In this test a gdbserver process is started and then, after gdbserver > has started, but before GDB attaches, we either delete the inferior > executable, or change its permissions so it can't be read. Only after > doing this do we attempt to connect with GDB. > > As GDB connects to gdbserver, gdbserver attempts to figure out the > target description so that it can send the description to GDB, this > involves a call to x86_linux_read_description. > > In x86_linux_read_description one of the first things we do is try to > figure out if the process is 32-bit or 64-bit. To do this we look up > the executable via the thread-id, and then attempt to read the > architecture size from the executable. This isn't going to work if > the executable has been deleted, or is no longer readable. > > And so, as we can't read the executable, we default to an i386 target > and use an i386 target description. > > A consequence of using an i386 target description is that addresses > are assumed to be 32-bits. Here's an example session that shows the > problems this causes. This is run on an x86-64 machine, and the test > binary (xx.x) is a standard 64-bit x86-64 binary: > > shell_1$ gdbserver --once localhost :54321 /tmp/xx.x > > shell_2$ gdb -q > (gdb) set sysroot > (gdb) shell chmod 000 /tmp/xx.x > (gdb) target remote :54321 > Remote debugging using :54321 > warning: /tmp/xx.x: Permission denied. > 0xf7fd3110 in ?? () > (gdb) show architecture > The target architecture is set to "auto" (currently "i386"). > (gdb) p/x $pc > $1 = 0xf7fd3110 > (gdb) info proc mappings > process 2412639 > Mapped address spaces: > > Start Addr End Addr Size Offset Perms objfile > 0x400000 0x401000 0x1000 0x0 r--p /tmp/xx.x > 0x401000 0x402000 0x1000 0x1000 r-xp /tmp/xx.x > 0x402000 0x403000 0x1000 0x2000 r--p /tmp/xx.x > 0x403000 0x405000 0x2000 0x2000 rw-p /tmp/xx.x > 0xf7fcb000 0xf7fcf000 0x4000 0x0 r--p [vvar] > 0xf7fcf000 0xf7fd1000 0x2000 0x0 r-xp [vdso] > 0xf7fd1000 0xf7fd3000 0x2000 0x0 r--p /usr/lib64/ld-2.30.so > 0xf7fd3000 0xf7ff3000 0x20000 0x2000 r-xp /usr/lib64/ld-2.30.so > 0xf7ff3000 0xf7ffb000 0x8000 0x22000 r--p /usr/lib64/ld-2.30.so > 0xf7ffc000 0xf7ffe000 0x2000 0x2a000 rw-p /usr/lib64/ld-2.30.so > 0xf7ffe000 0xf7fff000 0x1000 0x0 rw-p > 0xfffda000 0xfffff000 0x25000 0x0 rw-p [stack] > 0xff600000 0xff601000 0x1000 0x0 r-xp [vsyscall] > (gdb) info inferiors > Num Description Connection Executable > * 1 process 2412639 1 (remote :54321) > (gdb) shell cat /proc/2412639/maps > 00400000-00401000 r--p 00000000 fd:03 45907133 /tmp/xx.x > 00401000-00402000 r-xp 00001000 fd:03 45907133 /tmp/xx.x > 00402000-00403000 r--p 00002000 fd:03 45907133 /tmp/xx.x > 00403000-00405000 rw-p 00002000 fd:03 45907133 /tmp/xx.x > 7ffff7fcb000-7ffff7fcf000 r--p 00000000 00:00 0 [vvar] > 7ffff7fcf000-7ffff7fd1000 r-xp 00000000 00:00 0 [vdso] > 7ffff7fd1000-7ffff7fd3000 r--p 00000000 fd:00 143904 /usr/lib64/ld-2.30.so > 7ffff7fd3000-7ffff7ff3000 r-xp 00002000 fd:00 143904 /usr/lib64/ld-2.30.so > 7ffff7ff3000-7ffff7ffb000 r--p 00022000 fd:00 143904 /usr/lib64/ld-2.30.so > 7ffff7ffc000-7ffff7ffe000 rw-p 0002a000 fd:00 143904 /usr/lib64/ld-2.30.so > 7ffff7ffe000-7ffff7fff000 rw-p 00000000 00:00 0 > 7ffffffda000-7ffffffff000 rw-p 00000000 00:00 0 [stack] > ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall] > (gdb) > > Notice the difference between the mappings reported via GDB and those > reported directly from the kernel via /proc/PID/maps, the addresses of > every mapping is clamped to 32-bits for GDB, while the kernel reports > real 64-bit addresses. > > Notice also that the $pc value is a 32-bit value. It appears to be > within one of the mappings reported by GDB, but is outside any of the > mappings reported from the kernel. > > And this is where the problem arises. When gdbserver detaches from > the inferior we pass the inferior the address from which it should > resume. Due to the 32/64 bit confusion we tell the inferior to resume > from the 32-bit $pc value, which is not within any valid mapping, and > so, as soon as the inferior resumes, it segfaults. > > If we look at how GDB (not gdbserver) figures out its target > description then we see an interesting difference. GDB doesn't try to > read the executable. Instead GDB uses ptrace to query the thread's > state, and uses this to figure out the if the thread is 32 or 64 bit. > > If we update gdbserver to do it the "GDB" way then the above problem > is resolved, gdbserver now sees the process as 64-bit, and when we > detach from the inferior we give it the correct 64-bit address, and > the inferior no longer segfaults. > > Now, I could just update the gdbserver code, but better, I think, to > share one copy of the code between GDB and gdbserver in gdb/nat/. > That is what this commit does. > > The cores of x86_linux_read_description from gdbserver and > x86_linux_nat_target::read_description from GDB are moved into a new > file gdb/nat/x86-linux-tdesc.c and combined into a single function > x86_linux_tdesc_for_tid which is called from each location. > > This new function does things mostly the GDB way, some changes are > needed to allow for the sharing; we now take some pointers for where > the shared code can cache the xcr0 and xsave layout values. > > Another thing to note about this commit is how the functions > i386_linux_read_description and amd64_linux_read_description are > handled. For now I've left these function as implemented separately > in GDB and gdbserver. I've moved the declarations of these functions > into gdb/arch/{i386,amd64}-linux-tdesc.h, but the implementations are > left where they are. > > A later commit in this series will make these functions shared too, > but doing this is not trivial, so I've left that for a separate > commit. Merging the declarations as I've done here ensures that > everyone implements the function to the same API, and once these > functions are shared (in a later commit) we'll want a shared > declaration anyway. > > Reviewed-By: Felix Willgerodt Acked-By: John Baldwin -- John Baldwin