Re: Concerns regarding e17bebd049 ("dump: Set correct vaddr for ELF dump")

[Laszlo Ersek]

On 9/20/23 19:35, Stephen Brennan wrote:
> Hi Jon,
> 
> Jon Doron <arilou@gmail.com> writes:
>> Hi Stephen,
>> Like you have said the reason is as I wrote in the commit message, 
>> without "fixing" the vaddr GDB is messing up mapping and working with 
>> the generated core file.
> 
> For the record I totally love this workaround :)
> 
> It's clever and gets the job done and I would have done it in a
> heartbeat. It's just that it does end up making vmcores that have
> incorrect data, which is a pain for debuggers that are actually designed
> to look at kernel core dumps.
> 
>> This patch is almost 4 years old, perhaps some changes to GDB has been 
>> introduced to resolve this, I have not checked since then.
> 
> Program Headers:
>   Type           Offset             VirtAddr           PhysAddr
>                  FileSiz            MemSiz              Flags  Align
>   NOTE           0x0000000000000168 0x0000000000000000 0x0000000000000000
>                  0x0000000000001980 0x0000000000001980         0x0
>   LOAD           0x0000000000001ae8 0x0000000000000000 0x0000000000000000
>                  0x0000000080000000 0x0000000080000000         0x0
>   LOAD           0x0000000080001ae8 0x0000000000000000 0x00000000fffc0000
>                  0x0000000000040000 0x0000000000040000         0x0
> 
> (gdb) info files
> Local core dump file:
>         `/home/stepbren/repos/test_code/elf/dumpfile', file type elf64-x86-64.
>         0x0000000000000000 - 0x0000000080000000 is load1
>         0x0000000000000000 - 0x0000000000040000 is load2
> 
> $ gdb --version 
> GNU gdb (GDB) Red Hat Enterprise Linux 10.2-10.0.2.el9
> Copyright (C) 2021 Free Software Foundation, Inc.
> License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
> This is free software: you are free to change and redistribute it.
> There is NO WARRANTY, to the extent permitted by law.
> 
> 
> It doesn't *look like* anything has changed in this version of GDB. But
> I'm not really certain that GDB is expected to use the physical
> addresses in the load segments: it's not a kernel debugger.

The paging=off vmcores dumped by QEMU are primarily meant for the
"crash" utility <https://github.com/crash-utility/crash.git>, not gdb.
Crash builds upon gdb (it downloads a gdb tarball at build time, IIRC),
but either way, the vmcores are meant to be consumed by "crash", and
crash *is* a kernel debugger (both live, and post-mortem).

So, from my perspective: "whatever works with 'crash'". If you revert
Jon's commit and the vmcores continue working with "crash", I won't object.

I commented similarly under Jon's v1 patch -- as long as paging=off
dumps continue working with "crash", I'm fine:

http://mid.mail-archive.com/7961a154-f139-af73-613d-94b88bf95392@redhat.com

For reference, these are the v1 through v3 patch threads, from 2019:

http://mid.mail-archive.com/20181225125344.4482-1-arilou@gmail.com
http://mid.mail-archive.com/20190108130219.18550-1-arilou@gmail.com
http://mid.mail-archive.com/20190109082203.27142-1-arilou@gmail.com

Laszlo


> 
> I think hacking the p_vaddr field _is_ the way to get GDB to behave in
> the way you want: allow you to read physical memory addresses.
> 
>> As I'm no longer using this feature and have not worked and tested it 
>> in a long while, so I have no obligations to this change, but perhaps
>> someone else might be using it...
> 
> I definitely think it's valuable for people to continue being able to
> use QEMU vmcores generated with paging=off in GDB, even if GDB isn't
> desgined for it. It seems like a useful hack that appeals to the lowest
> common denominator: most people have GDB and not a purpose-built kernel
> debugger. But maybe we could point to a program like the below that will
> tweak the p_paddr field after the fact, in order to appeal to GDB's
> sensibilities?
> 
> Thanks,
> Stephen
> 
> ---
> #include <stdbool.h>
> #include <stdlib.h>
> #include <stdio.h>
> #include <string.h>
> #include <byteswap.h>
> 
> #include <elf.h>
> 
> static void fail(const char *msg)
> {
>       fprintf(stderr, "%s\n", msg);
>       exit(EXIT_FAILURE);
> }
> 
> static void perror_fail(const char *pfx)
> {
>       perror(pfx);
>       exit(EXIT_FAILURE);
> }
> 
> static void usage(void)
> {
>       puts("usage: phys2virt COREFILE");
>       puts("Modifies the ELF COREFILE so that load segments have their 
> virtual");
>       puts("address value copied from the physical address field.");
>       exit(EXIT_SUCCESS);
> }
> 
> static int endian(void)
> {
>       union {
>               uint32_t ival;
>               char     cval[4];
>       } data;
>       data.ival = 1;
>       if (data.cval[0])
>               return ELFDATA2LSB;
>       else
>               return ELFDATA2MSB;
> }
> 
> int main(int argc, char **argv)
> {
>       char *filename;
>       FILE *f;
>       Elf64_Ehdr hdr;
>       Elf64_Phdr *phdrs;
>       off_t phoff;
>       int phnum, phentsize;
>       bool bswap;
> 
>       if (argc != 2 || strcmp(argv[1], "-h") == 0)
>               usage();
> 
>       filename = argv[1];
>       f = fopen(filename,  "r+");
>       if (!f)
>               perror_fail("open");
> 
>       if (fread(&hdr, sizeof(hdr), 1, f) != 1)
>               perror_fail("read elf header");
> 
>       if (memcmp(hdr.e_ident, ELFMAG, 4) != 0)
>               fail("not an ELF file");
> 
>       if (hdr.e_ident[EI_CLASS] != ELFCLASS64)
>               fail("file is not 64-bits: unsupported");
> 
>       if (bswap) {
>               phoff = bswap_64(hdr.e_phoff);
>               phnum = bswap_16(hdr.e_phnum);
>               phentsize = bswap_16(hdr.e_phentsize);
>       } else {
>               phoff = hdr.e_phoff;
>               phnum = hdr.e_phnum;
>               phentsize = hdr.e_phentsize;
>       }
>       if (phentsize != sizeof(Elf64_Phdr))
>               fail("error: mismatch between phentsize and 
> sizeof(Elf64_Phdr)");
> 
>       if (fseek(f, phoff, SEEK_SET) < 0)
>               perror_fail("fseek");
> 
>       phdrs = calloc(phnum, phentsize);
>       if (!phdrs)
>               fail("error: allocation error");
> 
>       if (fread(phdrs, phentsize, phnum, f) != phnum)
>               perror_fail("fread phdrs");
> 
>       for (int i = 0; i < phnum; i++)
>               phdrs[i].p_vaddr = 0; //phdrs[i].p_paddr;
> 
>       if (fseek(f, phoff, SEEK_SET) < 0)
>               perror_fail("fseek");
>       if (fwrite(phdrs, phentsize, phnum, f) != phnum)
>               perror_fail("fwrite phdrs");
> 
>       fclose(f);
>       return EXIT_SUCCESS;
> }
>