On 11/02/16 at 04:00am, Thiago Jung Bauermann wrote: > Hello, > > The kexec_file code currently builds the purgatory as a partially linked > object > (using ld -r). Is there a particular reason to use that instead of a position > independent executable (PIE)?
It's taken as "-r", relocatable in user space kexec-tools too originally. I think Vivek just keeps it the same when moving into kernel. > > I found a discussion from 2013 in the archives but from what I understood it > was about the purgatory as a separate object vs having it linked into the > kernel, which is different from what I'm asking: > > http://lists.infradead.org/pipermail/kexec/2013-December/010535.html > > Here is my motivation for this question: > > On ppc64 purgatory.ro has 12 relocation types when built as a partially > linked object. This makes arch_kexec_apply_relocations_add duplicate a lot of > code with module_64.c:apply_relocate_add to implement these relocations. The > alternative is to do some refactoring so that both functions can share the > implementation of the relocations. This is done in patches 5 and 6 of the > kexec_file_load implementation for powerpc: In user space kexec-tools utility, you also got this problem? > > https://lists.ozlabs.org/pipermail/linuxppc-dev/2016-October/149984.html > > Michael Ellerman would prefer if module_64.c didn't need to be changed, and > suggested that the purgatory could be a position independent executable. > Indeed, in that case there are only 4 relocation types in purgatory.ro (which > aren't even implemented in module_64.c:apply_relocate_add), so the relocation > code for the purgatory can leave that file alone and have its own relocation > implementation. > > Also, the purgatory is an executable and not an intermediary output from the > compiler, so in my mind it makes sense conceptually that it is easier to > build > it as a PIE than as a partially linked object. > > The patch below adds the support needed in kexec_file.c to allow powerpc- > specific code to load and relocate a purgatory binary built as PIE. This is > WIP > and can probably be refined a bit. Would you accept a change along these > lines? > > Signed-off-by: Thiago Jung Bauermann <bauer...@linux.vnet.ibm.com> > --- > arch/Kconfig | 3 + > kernel/kexec_file.c | 159 > ++++++++++++++++++++++++++++++++++++++++++++++-- > kernel/kexec_internal.h | 26 ++++++++ > 3 files changed, 183 insertions(+), 5 deletions(-) > > diff --git a/arch/Kconfig b/arch/Kconfig > index 659bdd079277..7fd6879be222 100644 > --- a/arch/Kconfig > +++ b/arch/Kconfig > @@ -5,6 +5,9 @@ > config KEXEC_CORE > bool > > +config HAVE_KEXEC_FILE_PIE_PURGATORY > + bool > + > config OPROFILE > tristate "OProfile system profiling" > depends on PROFILING > diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c > index 0c2df7f73792..dfc3e015160d 100644 > --- a/kernel/kexec_file.c > +++ b/kernel/kexec_file.c > @@ -633,7 +633,149 @@ static int kexec_calculate_store_digests(struct kimage > *image) > return ret; > } > > -/* Actually load purgatory. Lot of code taken from kexec-tools */ > +#ifdef CONFIG_HAVE_KEXEC_FILE_PIE_PURGATORY > +/* Load PIE purgatory using the program header information. */ > +static int __kexec_load_purgatory(struct kimage *image, unsigned long min, > + unsigned long max, int top_down) > +{ > + struct purgatory_info *pi = &image->purgatory_info; > + unsigned long first_offset; > + unsigned long orig_load_addr = 0; > + const void *src; > + int i, ret; > + const Elf_Phdr *phdrs = (const void *) pi->ehdr + pi->ehdr->e_phoff; > + const Elf_Phdr *phdr; > + const Elf_Shdr *sechdrs_c; > + Elf_Shdr *sechdr; > + Elf_Shdr *sechdrs = NULL; > + struct kexec_buf kbuf = { .image = image, .bufsz = 0, .buf_align = 1, > + .buf_min = min, .buf_max = max, > + .top_down = top_down }; > + > + /* > + * sechdrs_c points to section headers in purgatory and are read > + * only. No modifications allowed. > + */ > + sechdrs_c = (void *) pi->ehdr + pi->ehdr->e_shoff; > + > + /* > + * We can not modify sechdrs_c[] and its fields. It is read only. > + * Copy it over to a local copy where one can store some temporary > + * data and free it at the end. We need to modify ->sh_addr and > + * ->sh_offset fields to keep track of permanent and temporary > + * locations of sections. > + */ > + sechdrs = vzalloc(pi->ehdr->e_shnum * sizeof(Elf_Shdr)); > + if (!sechdrs) > + return -ENOMEM; > + > + memcpy(sechdrs, sechdrs_c, pi->ehdr->e_shnum * sizeof(Elf_Shdr)); > + > + /* > + * We seem to have multiple copies of sections. First copy is which > + * is embedded in kernel in read only section. Some of these sections > + * will be copied to a temporary buffer and relocated. And these > + * sections will finally be copied to their final destination at > + * segment load time. > + * > + * Use ->sh_offset to reflect section address in memory. It will > + * point to original read only copy if section is not allocatable. > + * Otherwise it will point to temporary copy which will be relocated. > + * > + * Use ->sh_addr to contain final address of the section where it > + * will go during execution time. > + */ > + for (sechdr = sechdrs; sechdr < sechdrs + pi->ehdr->e_shnum; sechdr++) { > + if (sechdr->sh_type == SHT_NOBITS) > + continue; > + > + sechdr->sh_offset = (unsigned long) pi->ehdr + > sechdr->sh_offset; > + } > + > + /* Determine how much memory is needed to load the executable. */ > + for (phdr = phdrs; phdr < phdrs + pi->ehdr->e_phnum; phdr++) { > + if (phdr->p_type != PT_LOAD) > + continue; > + > + if (!orig_load_addr) { > + orig_load_addr = phdr->p_vaddr - phdr->p_offset; > + kbuf.bufsz = first_offset = phdr->p_offset; > + } > + > + if (kbuf.buf_align < phdr->p_align) { > + pr_debug("buf_align was %lx, now is %llx\n", > + kbuf.buf_align, phdr->p_align); > + kbuf.buf_align = phdr->p_align; > + } > + > + kbuf.bufsz += phdr->p_memsz; > + } > + > + /* Allocate buffer for purgatory. */ > + kbuf.buffer = vzalloc(kbuf.bufsz); > + if (!kbuf.buffer) { > + ret = -ENOMEM; > + goto out; > + } > + > + /* Add buffer to segment list. */ > + kbuf.memsz = kbuf.bufsz; > + ret = kexec_add_buffer(&kbuf); > + if (ret) > + goto out; > + > + pi->purgatory_load_addr = kbuf.mem; > + > + /* Load executable. */ > + for (phdr = phdrs; phdr < phdrs + pi->ehdr->e_phnum; phdr++) { > + if (phdr->p_type != PT_LOAD) > + continue; > + > + src = (const void *) pi->ehdr + phdr->p_offset; > + memcpy(kbuf.buffer + phdr->p_offset, src, phdr->p_filesz); > + > + pr_debug("loaded segment of size %llx at %llx (base = %lx, > offset = %llx)\n", > + phdr->p_memsz, pi->purgatory_load_addr + > phdr->p_offset, pi->purgatory_load_addr, phdr->p_offset); > + > + /* > + * Find sections within this segment and update their > + * ->sh_offset to point to within the buffer. > + */ > + for (i = 0; i < pi->ehdr->e_shnum; i++) { > + if (sechdrs[i].sh_addr >= phdr->p_vaddr && > + sechdrs[i].sh_addr + sechdrs[i].sh_size <= > phdr->p_vaddr + phdr->p_memsz) { > + sechdrs[i].sh_addr = sechdrs[i].sh_addr - > orig_load_addr + pi->purgatory_load_addr; > + sechdrs[i].sh_offset = (unsigned long long) > kbuf.buffer + sechdrs_c[i].sh_offset; > + } > + } > + } > + > + /* Make kernel jump to purgatory after shutdown */ > + image->start = pi->ehdr->e_entry - orig_load_addr + > pi->purgatory_load_addr; > + > + /* Used later to get/set symbol values */ > + pi->sechdrs = sechdrs; > + > + /* > + * Used later to identify which section is purgatory and skip it > + * from checksumming. > + */ > + pi->purgatory_buf = kbuf.buffer; > + > + pr_debug("purgatory entry point at %lx\n", image->start); > + > + return 0; > +out: > + vfree(sechdrs); > + vfree(kbuf.buffer); > + > + return ret; > +} > +#else /* CONFIG_HAVE_KEXEC_FILE_PIE_PURGATORY */ > +/* > + * Load relocatable object purgatory using the section header information. > + * A lot of code taken from kexec-tools. > + */ > static int __kexec_load_purgatory(struct kimage *image, unsigned long min, > unsigned long max, int top_down) > { > @@ -813,6 +955,7 @@ static int __kexec_load_purgatory(struct kimage *image, > unsigned long min, > vfree(kbuf.buffer); > return ret; > } > +#endif /* CONFIG_HAVE_KEXEC_FILE_PIE_PURGATORY */ > > static int kexec_apply_relocations(struct kimage *image) > { > @@ -886,7 +1029,7 @@ int kexec_load_purgatory(struct kimage *image, unsigned > long min, > pi->ehdr = (Elf_Ehdr *)kexec_purgatory; > > if (memcmp(pi->ehdr->e_ident, ELFMAG, SELFMAG) != 0 > - || pi->ehdr->e_type != ET_REL > + || pi->ehdr->e_type != PURGATORY_ELF_TYPE > || !elf_check_arch(pi->ehdr) > || pi->ehdr->e_shentsize != sizeof(Elf_Shdr)) > return -ENOEXEC; > @@ -942,7 +1085,13 @@ static Elf_Sym *kexec_purgatory_find_symbol(struct > purgatory_info *pi, > > /* Go through symbols for a match */ > for (k = 0; k < sechdrs[i].sh_size/sizeof(Elf_Sym); k++) { > - if (ELF_ST_BIND(syms[k].st_info) != STB_GLOBAL) > + /* > + * FIXME: See if we can or should export the .TOC. > + * symbol as global instead of searching local symbols > + * here. > + */ > + if (ELF_ST_BIND(syms[k].st_info) != STB_GLOBAL && > + ELF_ST_BIND(syms[k].st_info) != STB_LOCAL) > continue; > > if (strcmp(strtab + syms[k].st_name, name) != 0) > @@ -979,7 +1128,7 @@ void *kexec_purgatory_get_symbol_addr(struct kimage > *image, const char *name) > * Returns the address where symbol will finally be loaded after > * kexec_load_segment() > */ > - return (void *)(sechdr->sh_addr + sym->st_value); > + return (void *)(sechdr->sh_addr + sym_value_offset(pi, sym)); > } > > /* > @@ -1013,7 +1162,7 @@ int kexec_purgatory_get_set_symbol(struct kimage > *image, const char *name, > } > > sym_buf = (unsigned char *)sechdrs[sym->st_shndx].sh_offset + > - sym->st_value; > + sym_value_offset(pi, sym); > > if (get_value) > memcpy((void *)buf, sym_buf, size); > diff --git a/kernel/kexec_internal.h b/kernel/kexec_internal.h > index 4cef7e4706b0..c253b00f88d0 100644 > --- a/kernel/kexec_internal.h > +++ b/kernel/kexec_internal.h > @@ -20,6 +20,32 @@ struct kexec_sha_region { > unsigned long len; > }; > > +#ifdef CONFIG_HAVE_KEXEC_FILE_PIE_PURGATORY > +#define PURGATORY_ELF_TYPE ET_EXEC > + > +/* > + * In position-independent executables, the symbol value is an absolute > address, > + * so convert it to a section-relative offset. > + */ > +static inline Elf_Addr sym_value_offset(struct purgatory_info *pi, Elf_Sym > *sym) > +{ > + const Elf_Shdr *sechdrs_c = (const void *) pi->ehdr + pi->ehdr->e_shoff; > + > + return sym->st_value - sechdrs_c[sym->st_shndx].sh_addr; > +} > +#else > +#define PURGATORY_ELF_TYPE ET_REL > + > +/* > + * In a relocatable object, the symbol value already is a section-relative > + * offset. > + */ > +static inline Elf_Addr sym_value_offset(struct purgatory_info *pi, Elf_Sym > *sym) > +{ > + return sym->st_value; > +} > +#endif > + > void kimage_file_post_load_cleanup(struct kimage *image); > #else /* CONFIG_KEXEC_FILE */ > static inline void kimage_file_post_load_cleanup(struct kimage *image) { } > -- > 2.7.4 > > > > _______________________________________________ > kexec mailing list > ke...@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/kexec
