Re: [Crash-utility] [PATCH] filesys: Skip 'vmlinux.o' while searching for vmlinux in 'find_booted_kernel()'

Hi Bhupesh, Thanks -- mystery solved! I kind of hate to simply use the file name string as the qualifier, though. Looking at the ELF headers of the vmlinux.o and vmlinux file, there are several notable differences: $ readelf -a vmlinux ELF Header: Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 Class: ELF64 Data: 2's complement, little endian Version: 1 (current) OS/ABI: UNIX - System V ABI Version: 0 Type: EXEC (Executable file) Machine: AArch64 Version: 0x1 Entry point address: 0xffff000008080000 Start of program headers: 64 (bytes into file) Start of section headers: 186677936 (bytes into file) Flags: 0x0 Size of this header: 64 (bytes) Size of program headers: 56 (bytes) Number of program headers: 4 Size of section headers: 64 (bytes) Number of section headers: 42 Section header string table index: 41 ... $ readelf -a vmlinux.o ELF Header: Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 Class: ELF64 Data: 2's complement, little endian Version: 1 (current) OS/ABI: UNIX - System V ABI Version: 0 Type: REL (Relocatable file) Machine: AArch64 Version: 0x1 Entry point address: 0x0 Start of program headers: 0 (bytes into file) Start of section headers: 478638376 (bytes into file) Flags: 0x0 Size of this header: 64 (bytes) Size of program headers: 0 (bytes) Number of program headers: 0 Size of section headers: 64 (bytes) Number of section headers: 34816 Section header string table index: 34815 ... There is the "Type", the "Entry point address", and perhaps most notably, the vmlinux.o's "Number of program headers:" count of 0, which obviously would preclude the file from being usable.

I would prefer to beef up the currently-existing is_kernel() function, and use it instead of is_elf_file(). Let me take a look at what works.

----- Original Message ----- > I found this particular issue while compiling and installing > a kernel from source on a RHEL arm64 machine. I used the following > commands to build and install the kernel from source: > > # make olddefconfig > # make > # make modules_install INSTALL_MOD_STRIP=1 && make install > > This compiles and installs both 'vmlinux' and 'vmlinux.o' at > the following standard location - '/lib/modules/<kernel>/build/ > > Now the problem can be reproduced using the following steps: > 1. Update grub to boot the freshly compiled kernel. > 2. Reboot machine. > 3. Now, run crash on live system, it errors out with the following > messages: > > crash: invalid kernel virtual address: 8470 type: "possible" > WARNING: cannot read cpu_possible_map > crash: invalid kernel virtual address: 8270 type: "present" > WARNING: cannot read cpu_present_map > crash: invalid kernel virtual address: 8070 type: "online" > WARNING: cannot read cpu_online_map > crash: invalid kernel virtual address: 8670 type: "active" > WARNING: cannot read cpu_active_map > > crash: cannot resolve "_stext" > > 4. Enabling some debug messages in 'find_booted_kernel()' function > tells us that it finds 'vmlinux.o' earlier than 'vmlinux' and accepts > that as the kernel being boot'ed: > > find_booted_kernel: check: /lib/modules/4.14.0/build/vmlinux.o > find_booted_kernel: found: /lib/modules/4.14.0/build/vmlinux.o > > 5. Now the problem happens due to following check inside > 'find_booted_kernel()' function: > > if (mount_point(kernel) || > !file_readable(kernel) || > !is_elf_file(kernel)) > continue; > > 6. Since 'vmlinux.o' is a elf file, is readable and is not > mount'able, so the check in point 5 fails and we incorrectly > accept this as the kernel being boot'ed, even though > there was a 'vmlinux' present inside > '/lib/modules/<kernel>/build'. > > 7. Now, later when crash tries to access symbols (like _stext) > using this kernel symbol file, we see errors. > > Fix this by skipping 'vmlinux.o' from the check in point 5, > so that we can select 'vmlinux' correctly as the kernel file. > > After this fix, crash no longer errors out and we can use > full functionality on the crash prompt. > > Signed-off-by: Bhupesh Sharma <bhsharma(a)redhat.com&gt; > --- > filesys.c | 14 +++++++++++++- > 1 file changed, 13 insertions(+), 1 deletion(-) > > diff --git a/filesys.c b/filesys.c > index 1b44ad5cefa8..9f240d6115a1 100644 > --- a/filesys.c > +++ b/filesys.c > @@ -623,9 +623,21 @@ find_booted_kernel(void) > > sprintf(kernel, "%s%s", searchdirs[i], dp->d_name); > > + /* We may run into a special case with > + * 'vmlinux.o' been present inside > + * '/lib/modules/<kernel>/build/', > + * which has some special characteristics - > + * this is a elf file, is readable and is not > + * mount'able. > + * > + * So we need to handle this properly here to > + * make sure that we get to the real 'vmlinux' > + * rather than the 'vmlinux.o' > + */ > if (mount_point(kernel) || > !file_readable(kernel) || > - !is_elf_file(kernel)) > + !is_elf_file(kernel) || > + !(strcmp(dp->d_name, "vmlinux.o"))) > continue; > > if (CRASHDEBUG(1)) > -- > 2.7.4 > >