[Crash-utility] Re: [PATCH v2] Disable DT_DEBUG lookup by GDB inside the vmcore

Hi Shivang, Sorry for the late. LGTM, so ack. Thanks, Tao Liu On Mon, Jul 21, 2025 at 8:18 PM Shivang Upadhyay <shivangu(a)linux.ibm.com&gt; wrote: > > Crash with GDB 16.2, the following warnings are printed: > > crash> > crash: page excluded: kernel virtual address: c0000000022d6098 type: "gdb_readmem_callback" > crash: page excluded: kernel virtual address: c0000000022d6098 type: "gdb_readmem_callback" > > This occurs because the elf_locate_base function in GDB 16.2 > attempts to read the address of the dynamic linker runtime > structure, which is present in the .dynamic section of the > executable. However, this section may be excluded from the > dump by makedumpfile. > > The repeated calls to elf_locate_base were introduced by gdb > commit [1] aebb370 ("gdb, solib-svr4: support namespaces in > DSO iteration") via svr4_iterate_over_objfiles_in_search_order. > > To check whether the kernel includes DT_DEBUG information, > prints were added inside crash::xfer_partial, which is > called through elf_locate_base when reading from vmcore. > Even when running crash on /proc/kcore, all output data was > zero. This confirms that DT_DEBUG information is never > present in the kernel image. > > `mod -S` continues to function correctly after the following > patch: > > ... > crash> mod -S > Enable debuginfod for this session? (y or [n]) > MODULE NAME TEXT_BASE SIZE OBJECT FILE > c0080000004a0300 dm_log c008000000480000 196608 XXX/lib/modules/5.14.0-592.el9.ppc64le/kernel/drivers/md/dm-log.ko > c0080000006d1100 sd_mod c008000000580000 196608 XXX/lib/modules/5.14.0-592.el9.ppc64le/kernel/drivers/scsi/sd_mod.ko > c0080000005c0080 dm_region_hash c0080000005a0000 196608 XXX/lib/modules/5.14.0-592.el9.ppc64le/kernel/drivers/md/dm-region-hash.ko > c008000000770700 sg c008000000620000 262144 XXX/lib/modules/5.14.0-592.el9.ppc64le/kernel/drivers/scsi/sg.ko > c008000000660500 dm_mirror c008000000640000 196608 XXX/lib/modules/5.14.0-592.el9.ppc64le/kernel/drivers/md/dm-mirror.ko > ... > > > > Commit e906eaca2b1a ("Fix the issue of "page excluded" > messages flooding") attempted fix this by suppressing these > warnings for regular users, but the warnings still appear > when crash is started in debug mode. > > To fix this, remove the DT_DEBUG read call, from the > elf_locate_base function in GDB that tries to read the > .dynamic section, as this information is not useful for > debugging kernel images in either dump or live kernel > scenarios. > > > [1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=aebb370 > > Cc: Tao liu <ltao(a)redhat.com&gt; > Cc: Lianbo Jiang <lijiang(a)redhat.com&gt; > Cc: Sourabh Jain <sourabhjain(a)linux.ibm.com&gt; > Signed-off-by: shivang.upadhyay <shivangu(a)linux.ibm.com&gt; > --- > gdb-16.2.patch | 16 ++++++++++++++++ > 1 file changed, 16 insertions(+) > > diff --git a/gdb-16.2.patch b/gdb-16.2.patch > index 151e4e2..7b79cdf 100644 > --- a/gdb-16.2.patch > +++ b/gdb-16.2.patch > @@ -1952,3 +1952,19 @@ exit 0 > } > > /* Remember the bfd indexes for the .text, .data, .bss and > +--- gdb-16.2/gdb/solib-svr4.c.orig > ++++ gdb-16.2/gdb/solib-svr4.c > +@@ -742,11 +742,13 @@ elf_locate_base (void) > + return extract_typed_address (pbuf, ptr_type); > + } > + > ++#ifndef CRASH_MERGE > + /* Find DT_DEBUG. */ > + if (gdb_bfd_scan_elf_dyntag (DT_DEBUG, current_program_space->exec_bfd (), > + &dyn_ptr, NULL) > + || scan_dyntag_auxv (DT_DEBUG, &dyn_ptr, NULL)) > + return dyn_ptr; > ++#endif > + > + /* This may be a static executable. Look for the symbol > + conventionally named _r_debug, as a last resort. */ > -- > 2.50.0 >