On 9/20/23 19:35, Stephen Brennan wrote: > Hi Jon, > > Jon Doron <ari...@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; > } >