[v4,07/17] dump: Allocate header

Commit Message

Janosch Frank July 26, 2022, 9:22 a.m. UTC

Allocating the header lets us write it at a later time and hence also
allows us to change section and segment table offsets until we
finally write it.

Signed-off-by: Janosch Frank <[e-mail]>
Reviewed-by: Marc-André Lureau <[e-mail]>
---
 dump/dump.c           | 127 +++++++++++++++++++++---------------------
 include/sysemu/dump.h |   1 +
 2 files changed, 64 insertions(+), 64 deletions(-)

Comments

Janis Schoetterl-Glausch July 29, 2022, 6:31 p.m. UTC | #1

On 7/26/22 11:22, Janosch Frank wrote:
> Allocating the header lets us write it at a later time and hence also
> allows us to change section and segment table offsets until we
> finally write it.
> 
Where are you making use of this?
You set e_shstrndx in prepare_elf_section_hdrs, but that is not required, is it?
As far as I understand we know shdr_num after dump_init so that could be moved.

Patch

diff --git a/dump/dump.c b/dump/dump.c
index 57558a4d4b..8a7ce95610 100644
--- a/dump/dump.c
+++ b/dump/dump.c
@@ -98,6 +98,7 @@  static int dump_cleanup(DumpState *s)
     memory_mapping_list_free(&s->list);
     close(s->fd);
     g_free(s->guest_note);
+    g_free(s->elf_header);
     s->guest_note = NULL;
     if (s->resume) {
         if (s->detached) {
@@ -126,73 +127,49 @@  static int fd_write_vmcore(const void *buf, size_t size, void *opaque)
     return 0;
 }
 
-static void write_elf64_header(DumpState *s, Error **errp)
+static void prepare_elf64_header(DumpState *s)
 {
-    /*
-     * phnum in the elf header is 16 bit, if we have more segments we
-     * set phnum to PN_XNUM and write the real number of segments to a
-     * special section.
-     */
-    uint16_t phnum = MIN(s->phdr_num, PN_XNUM);
-    Elf64_Ehdr elf_header;
-    int ret;
+    uint16_t phnum = s->phdr_num >= PN_XNUM ? PN_XNUM : s->phdr_num;
+    Elf64_Ehdr *elf_header = s->elf_header;
 
-    memset(&elf_header, 0, sizeof(Elf64_Ehdr));
-    memcpy(&elf_header, ELFMAG, SELFMAG);
-    elf_header.e_ident[EI_CLASS] = ELFCLASS64;
-    elf_header.e_ident[EI_DATA] = s->dump_info.d_endian;
-    elf_header.e_ident[EI_VERSION] = EV_CURRENT;
-    elf_header.e_type = cpu_to_dump16(s, ET_CORE);
-    elf_header.e_machine = cpu_to_dump16(s, s->dump_info.d_machine);
-    elf_header.e_version = cpu_to_dump32(s, EV_CURRENT);
-    elf_header.e_ehsize = cpu_to_dump16(s, sizeof(elf_header));
-    elf_header.e_phoff = cpu_to_dump64(s, s->phdr_offset);
-    elf_header.e_phentsize = cpu_to_dump16(s, sizeof(Elf64_Phdr));
-    elf_header.e_phnum = cpu_to_dump16(s, phnum);
+    memcpy(elf_header, ELFMAG, SELFMAG);
+    elf_header->e_ident[EI_CLASS] = ELFCLASS64;
+    elf_header->e_ident[EI_DATA] = s->dump_info.d_endian;
+    elf_header->e_ident[EI_VERSION] = EV_CURRENT;
+    elf_header->e_type = cpu_to_dump16(s, ET_CORE);
+    elf_header->e_machine = cpu_to_dump16(s, s->dump_info.d_machine);
+    elf_header->e_version = cpu_to_dump32(s, EV_CURRENT);
+    elf_header->e_ehsize = cpu_to_dump16(s, sizeof(*elf_header));
+    elf_header->e_phoff = cpu_to_dump64(s, s->phdr_offset);
+    elf_header->e_phentsize = cpu_to_dump16(s, sizeof(Elf64_Phdr));
+    elf_header->e_phnum = cpu_to_dump16(s, phnum);
     if (s->shdr_num) {
-        elf_header.e_shoff = cpu_to_dump64(s, s->shdr_offset);
-        elf_header.e_shentsize = cpu_to_dump16(s, sizeof(Elf64_Shdr));
-        elf_header.e_shnum = cpu_to_dump16(s, s->shdr_num);
-    }
-
-    ret = fd_write_vmcore(&elf_header, sizeof(elf_header), s);
-    if (ret < 0) {
-        error_setg_errno(errp, -ret, "dump: failed to write elf header");
+        elf_header->e_shoff = cpu_to_dump64(s, s->shdr_offset);
+        elf_header->e_shentsize = cpu_to_dump16(s, sizeof(Elf64_Shdr));
+        elf_header->e_shnum = cpu_to_dump16(s, s->shdr_num);
     }
 }
 
-static void write_elf32_header(DumpState *s, Error **errp)
+static void prepare_elf32_header(DumpState *s)
 {
-    /*
-     * phnum in the elf header is 16 bit, if we have more segments we
-     * set phnum to PN_XNUM and write the real number of segments to a
-     * special section.
-     */
-    uint16_t phnum = MIN(s->phdr_num, PN_XNUM);
-    Elf32_Ehdr elf_header;
-    int ret;
+    uint16_t phnum = s->phdr_num >= PN_XNUM ? PN_XNUM : s->phdr_num;
+    Elf32_Ehdr *elf_header = s->elf_header;
 
-    memset(&elf_header, 0, sizeof(Elf32_Ehdr));
-    memcpy(&elf_header, ELFMAG, SELFMAG);
-    elf_header.e_ident[EI_CLASS] = ELFCLASS32;
-    elf_header.e_ident[EI_DATA] = s->dump_info.d_endian;
-    elf_header.e_ident[EI_VERSION] = EV_CURRENT;
-    elf_header.e_type = cpu_to_dump16(s, ET_CORE);
-    elf_header.e_machine = cpu_to_dump16(s, s->dump_info.d_machine);
-    elf_header.e_version = cpu_to_dump32(s, EV_CURRENT);
-    elf_header.e_ehsize = cpu_to_dump16(s, sizeof(elf_header));
-    elf_header.e_phoff = cpu_to_dump32(s, s->phdr_offset);
-    elf_header.e_phentsize = cpu_to_dump16(s, sizeof(Elf32_Phdr));
-    elf_header.e_phnum = cpu_to_dump16(s, phnum);
+    memcpy(elf_header, ELFMAG, SELFMAG);
+    elf_header->e_ident[EI_CLASS] = ELFCLASS32;
+    elf_header->e_ident[EI_DATA] = s->dump_info.d_endian;
+    elf_header->e_ident[EI_VERSION] = EV_CURRENT;
+    elf_header->e_type = cpu_to_dump16(s, ET_CORE);
+    elf_header->e_machine = cpu_to_dump16(s, s->dump_info.d_machine);
+    elf_header->e_version = cpu_to_dump32(s, EV_CURRENT);
+    elf_header->e_ehsize = cpu_to_dump16(s, sizeof(*elf_header));
+    elf_header->e_phoff = cpu_to_dump32(s, s->phdr_offset);
+    elf_header->e_phentsize = cpu_to_dump16(s, sizeof(Elf32_Phdr));
+    elf_header->e_phnum = cpu_to_dump16(s, phnum);
     if (s->shdr_num) {
-        elf_header.e_shoff = cpu_to_dump32(s, s->shdr_offset);
-        elf_header.e_shentsize = cpu_to_dump16(s, sizeof(Elf32_Shdr));
-        elf_header.e_shnum = cpu_to_dump16(s, s->shdr_num);
-    }
-
-    ret = fd_write_vmcore(&elf_header, sizeof(elf_header), s);
-    if (ret < 0) {
-        error_setg_errno(errp, -ret, "dump: failed to write elf header");
+        elf_header->e_shoff = cpu_to_dump32(s, s->shdr_offset);
+        elf_header->e_shentsize = cpu_to_dump16(s, sizeof(Elf32_Shdr));
+        elf_header->e_shnum = cpu_to_dump16(s, s->shdr_num);
     }
 }
 
@@ -528,6 +505,26 @@  static void write_elf_notes(DumpState *s, Error **errp)
     }
 }
 
+static void prepare_elf_header(DumpState *s)
+{
+    if (dump_is_64bit(s)) {
+        prepare_elf64_header(s);
+    } else {
+        prepare_elf32_header(s);
+    }
+}
+
+static void write_elf_header(DumpState *s, Error **errp)
+{
+    size_t size = dump_is_64bit(s) ? sizeof(Elf64_Ehdr) : sizeof(Elf32_Ehdr);
+    int ret;
+
+    ret = fd_write_vmcore(s->elf_header, size, s);
+    if (ret < 0) {
+        error_setg_errno(errp, -ret, "dump: failed to write elf header");
+    }
+}
+
 /* write elf header, PT_NOTE and elf note to vmcore. */
 static void dump_begin(DumpState *s, Error **errp)
 {
@@ -557,12 +554,11 @@  static void dump_begin(DumpState *s, Error **errp)
      * vmcore.
      */
 
-    /* write elf header to vmcore */
-    if (dump_is_64bit(s)) {
-        write_elf64_header(s, errp);
-    } else {
-        write_elf32_header(s, errp);
-    }
+    /* Write elf header to buffer */
+    prepare_elf_header(s);
+
+    /* Start to write stuff into file descriptor */
+    write_elf_header(s, errp);
     if (*errp) {
         return;
     }
@@ -1679,6 +1675,9 @@  static void dump_init(DumpState *s, int fd, bool has_format,
         goto cleanup;
     }
 
+    s->elf_header = g_malloc0(dump_is_64bit(s) ?
+                              sizeof(Elf64_Ehdr) : sizeof(Elf32_Ehdr));
+
     /*
      * The goal of this block is to (a) update the previously guessed
      * phys_base, (b) copy the guest note out of the guest.
diff --git a/include/sysemu/dump.h b/include/sysemu/dump.h
index 24ebb02660..7f9a848573 100644
--- a/include/sysemu/dump.h
+++ b/include/sysemu/dump.h
@@ -171,6 +171,7 @@  typedef struct DumpState {
     int64_t begin;             /* Start address of the chunk we want to dump */
     int64_t length;            /* Length of the dump we want to dump */
 
+    void *elf_header;
     uint8_t *note_buf;          /* buffer for notes */
     size_t note_buf_offset;     /* the writing place in note_buf */
     uint32_t nr_cpus;           /* number of guest's cpu */