Re: [PATCH v4 11/17] dump/dump: Add section string table support

[Marc-André Lureau]

Hi

On Tue, Jul 26, 2022 at 1:23 PM Janosch Frank <frankja@linux.ibm.com> wrote:
>
> As sections don't have a type like the notes do we need another way to
> determine their contents. The string table allows us to assign each
> section an identification string which architectures can then use to
> tag their sections with.
>
> There will be no string table if the architecture doesn't add custom
> sections which are introduced in a following patch.
>
> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
> ---
>  dump/dump.c           | 81 ++++++++++++++++++++++++++++++++++++++++++-
>  include/sysemu/dump.h |  1 +
>  2 files changed, 81 insertions(+), 1 deletion(-)
>
> diff --git a/dump/dump.c b/dump/dump.c
> index 3cf846d0a0..298a1e923f 100644
> --- a/dump/dump.c
> +++ b/dump/dump.c
> @@ -99,6 +99,7 @@ static int dump_cleanup(DumpState *s)
>      close(s->fd);
>      g_free(s->guest_note);
>      g_free(s->elf_header);
> +    g_array_unref(s->string_table_buf);
>      s->guest_note = NULL;
>      if (s->resume) {
>          if (s->detached) {
> @@ -357,21 +358,72 @@ static size_t prepare_elf_section_hdr_zero(DumpState *s)
>      return dump_is_64bit(s) ? sizeof(Elf64_Shdr) : sizeof(Elf32_Shdr);
>  }
>
> +static void write_elf_section_hdr_string(DumpState *s, void *buff)
> +{

Mildly annoyed that we use "write_" prefix for actually writing to the
fd, and sometimes to pre-fill (or "prepare_" structures). Would you
mind cleaning that up?

> +    Elf32_Shdr shdr32;
> +    Elf64_Shdr shdr64;
> +    int shdr_size;
> +    void *shdr = buff;

Why assign here?

> +
> +    if (dump_is_64bit(s)) {
> +        shdr_size = sizeof(Elf64_Shdr);
> +        memset(&shdr64, 0, shdr_size);
> +        shdr64.sh_type = SHT_STRTAB;
> +        shdr64.sh_offset = s->section_offset + s->elf_section_data_size;
> +        shdr64.sh_name = s->string_table_buf->len;
> +        g_array_append_vals(s->string_table_buf, ".strtab", 
> sizeof(".strtab"));
> +        shdr64.sh_size = s->string_table_buf->len;
> +        shdr = &shdr64;
> +    } else {
> +        shdr_size = sizeof(Elf32_Shdr);
> +        memset(&shdr32, 0, shdr_size);
> +        shdr32.sh_type = SHT_STRTAB;
> +        shdr32.sh_offset = s->section_offset + s->elf_section_data_size;
> +        shdr32.sh_name = s->string_table_buf->len;
> +        g_array_append_vals(s->string_table_buf, ".strtab", 
> sizeof(".strtab"));
> +        shdr32.sh_size = s->string_table_buf->len;
> +        shdr = &shdr32;
> +    }
> +
> +    memcpy(buff, shdr, shdr_size);
> +}
> +
>  static void prepare_elf_section_hdrs(DumpState *s)
>  {
>      size_t len, sizeof_shdr;
> +    Elf64_Ehdr *hdr64 = s->elf_header;
> +    Elf32_Ehdr *hdr32 = s->elf_header;
> +    void *buff_hdr;
>
>      /*
>       * Section ordering:
>       * - HDR zero (if needed)
> +     * - String table hdr
>       */
>      sizeof_shdr = dump_is_64bit(s) ? sizeof(Elf64_Shdr) : sizeof(Elf32_Shdr);
>      len = sizeof_shdr * s->shdr_num;
>      s->elf_section_hdrs = g_malloc0(len);
> +    buff_hdr = s->elf_section_hdrs;
>
>      /* Write special section first */
>      if (s->phdr_num == PN_XNUM) {
>          prepare_elf_section_hdr_zero(s);
> +        buff_hdr += sizeof_shdr;
> +    }
> +
> +    if (s->shdr_num < 2) {
> +        return;
> +    }
> +
> +    /*
> +     * String table needs to be last section since strings are added
> +     * via arch_sections_write_hdr().
> +     */

This comment isn't exactly relevant yet, since that code isn't there, but ok.

> +    write_elf_section_hdr_string(s, buff_hdr);
> +    if (dump_is_64bit(s)) {
> +        hdr64->e_shstrndx = cpu_to_dump16(s, s->shdr_num - 1);
> +    } else {
> +        hdr32->e_shstrndx = cpu_to_dump16(s, s->shdr_num - 1);
>      }
>  }
>
> @@ -401,11 +453,18 @@ static void write_elf_sections(DumpState *s, Error 
> **errp)
>  {
>      int ret;
>
> -    /* Write section zero */
> +    /* Write section zero and arch sections */
>      ret = fd_write_vmcore(s->elf_section_data, s->elf_section_data_size, s);
>      if (ret < 0) {
>          error_setg_errno(errp, -ret, "dump: failed to write section data");
>      }
> +
> +    /* Write string table data */
> +    ret = fd_write_vmcore(s->string_table_buf->data,
> +                          s->string_table_buf->len, s);
> +    if (ret < 0) {
> +        error_setg_errno(errp, -ret, "dump: failed to write string table 
> data");
> +    }
>  }
>
>  static void write_data(DumpState *s, void *buf, int length, Error **errp)
> @@ -713,6 +772,7 @@ static void create_vmcore(DumpState *s, Error **errp)
>          return;
>      }
>
> +    /* Iterate over memory and dump it to file */
>      dump_iterate(s, errp);
>      if (*errp) {
>          return;
> @@ -1695,6 +1755,13 @@ static void dump_init(DumpState *s, int fd, bool 
> has_format,
>      s->has_filter = has_filter;
>      s->begin = begin;
>      s->length = length;
> +    /* First index is 0, it's the special null name */
> +    s->string_table_buf = g_array_new(FALSE, TRUE, 1);
> +    /*
> +     * Allocate the null name, due to the clearing option set to true
> +     * it will be 0.
> +     */
> +    g_array_set_size(s->string_table_buf, 1);

I wonder if GByteArray wouldn't be more appropriate, even if it
doesn't have the clearing option. If it's just for one byte, ...

>
>      memory_mapping_list_init(&s->list);
>
> @@ -1855,6 +1922,18 @@ static void dump_init(DumpState *s, int fd, bool 
> has_format,
>          }
>      }
>
> +    /*
> +     * calculate shdr_num and elf_section_data_size so we know the offsets 
> and
> +     * sizes of all parts.
> +     *
> +     * If phdr_num overflowed we have at least one section header
> +     * More sections/hdrs can be added by the architectures
> +     */
> +    if (s->shdr_num > 1) {
> +        /* Reserve the string table */
> +        s->shdr_num += 1;
> +    }
> +
>      tmp = (s->phdr_num == PN_XNUM) ? s->sh_info : s->phdr_num;
>      if (dump_is_64bit(s)) {
>          s->shdr_offset = sizeof(Elf64_Ehdr);
> diff --git a/include/sysemu/dump.h b/include/sysemu/dump.h
> index 696e6c67d6..299b611180 100644
> --- a/include/sysemu/dump.h
> +++ b/include/sysemu/dump.h
> @@ -178,6 +178,7 @@ typedef struct DumpState {
>      void *elf_section_hdrs;
>      uint64_t elf_section_data_size;
>      void *elf_section_data;
> +    GArray *string_table_buf;  /* String table section */
>
>      uint8_t *note_buf;          /* buffer for notes */
>      size_t note_buf_offset;     /* the writing place in note_buf */
> --
> 2.34.1
>