From dd09fe0d53242a5f6a86d2822b0cfdeb3f5baa8f Mon Sep 17 00:00:00 2001 From: Keith Seitz Date: Tue, 26 Jul 2022 19:11:04 +0100 Subject: [PATCH] gdb/linux_nat: Write memory using ptrace if /proc/pid/mem is not writable Commit 05c06f318fd9a112529dfc313e6512b399a645e4 enabled GDB to access memory while threads are running. It did this by accessing /proc/PID/task/LWP/mem. Unfortunately, this interface is not implemented for writing in older kernels (such as RHEL6). This means that GDB is unable to insert breakpoints on these hosts: $ ./gdb -q gdb -ex start Reading symbols from gdb... Temporary breakpoint 1 at 0x40fdd5: file ../../src/gdb/gdb.c, line 28. Starting program: /home/rhel6/fsf/linux/gdb/gdb Warning: Cannot insert breakpoint 1. Cannot access memory at address 0x40fdd5 (gdb) Before this patch, linux_proc_xfer_memory_partial (previously called linux_proc_xfer_partial) would return TARGET_XFER_EOF if the write to /proc/PID/mem failed. [More specifically, linux_proc_xfer_partial would not "bother for one word," but the effect is the essentially same.] This status was checked by linux_nat_target::xfer_partial, which would then fallback to using ptrace to perform the operation. This is the specific hunk that removed the fallback: - xfer = linux_proc_xfer_partial (object, annex, readbuf, writebuf, - offset, len, xfered_len); - if (xfer != TARGET_XFER_EOF) - return xfer; + return linux_proc_xfer_memory_partial (readbuf, writebuf, + offset, len, xfered_len); + } return inf_ptrace_target::xfer_partial (object, annex, readbuf, writebuf, offset, len, xfered_len); This patch makes linux_nat_target::xfer_partial go straight to writing memory via ptrace if writing via /proc/pid/mem is not possible in the running kernel, enabling GDB to insert breakpoints on these older kernels. Note that a recent patch changed the return status from TARGET_XFER_EOF to TARGET_XFER_E_IO. Tested on {unix,native-gdbserver,native-extended-gdbserver}/-m{32,64} on x86_64, s390x, aarch64, and ppc64le. Change-Id: If1d884278e8c4ea71d8836bedd56e6a6c242a415 --- gdb/linux-nat.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c index b641e88b1ef..e638e8ad04e 100644 --- a/gdb/linux-nat.c +++ b/gdb/linux-nat.c @@ -3711,8 +3711,15 @@ linux_nat_target::xfer_partial (enum target_object object, if (addr_bit < (sizeof (ULONGEST) * HOST_CHAR_BIT)) offset &= ((ULONGEST) 1 << addr_bit) - 1; - return linux_proc_xfer_memory_partial (readbuf, writebuf, - offset, len, xfered_len); + /* If /proc/pid/mem is writable, don't fallback to ptrace. If + the write via /proc/pid/mem fails because the inferior execed + (and we haven't seen the exec event yet), a subsequent ptrace + poke would incorrectly write memory to the post-exec address + space, while the core was trying to write to the pre-exec + address space. */ + if (proc_mem_file_is_writable ()) + return linux_proc_xfer_memory_partial (readbuf, writebuf, + offset, len, xfered_len); } return inf_ptrace_target::xfer_partial (object, annex, readbuf, writebuf,