dump: Use a buffer for ELF section data and headers

Currently we're writing the NULL section header if we overflow the
physical header number in the ELF header. But in the future we'll add
custom section headers AND section data.

To facilitate this we need to rearange section handling a bit. As with
the other ELF headers we split the code into a prepare and a write
step.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20221017083822.43118-2-frankja@linux.ibm.com>

dump/dump.c

@@ -380,31 +380,60 @@ static void write_elf_phdr_note(DumpState *s, Error **errp)
     }
 }
 
-static void write_elf_section(DumpState *s, int type, Error **errp)
+static void prepare_elf_section_hdr_zero(DumpState *s)
 {
-    Elf32_Shdr shdr32;
-    Elf64_Shdr shdr64;
-    int shdr_size;
-    void *shdr;
+    if (dump_is_64bit(s)) {
+        Elf64_Shdr *shdr64 = s->elf_section_hdrs;
+
+        shdr64->sh_info = cpu_to_dump32(s, s->phdr_num);
+    } else {
+        Elf32_Shdr *shdr32 = s->elf_section_hdrs;
+
+        shdr32->sh_info = cpu_to_dump32(s, s->phdr_num);
+    }
+}
+
+static void prepare_elf_section_hdrs(DumpState *s)
+{
+    size_t len, sizeof_shdr;
+
+    /*
+     * Section ordering:
+     * - HDR zero
+     */
+    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);
+
+    /*
+     * The first section header is ALWAYS a special initial section
+     * header.
+     *
+     * The header should be 0 with one exception being that if
+     * phdr_num is PN_XNUM then the sh_info field contains the real
+     * number of segment entries.
+     *
+     * As we zero allocate the buffer we will only need to modify
+     * sh_info for the PN_XNUM case.
+     */
+    if (s->phdr_num >= PN_XNUM) {
+        prepare_elf_section_hdr_zero(s);
+    }
+}
+
+static void write_elf_section_headers(DumpState *s, Error **errp)
+{
+    size_t sizeof_shdr = dump_is_64bit(s) ? sizeof(Elf64_Shdr) : sizeof(Elf32_Shdr);
     int ret;
 
-    if (type == 0) {
-        shdr_size = sizeof(Elf32_Shdr);
-        memset(&shdr32, 0, shdr_size);
-        shdr32.sh_info = cpu_to_dump32(s, s->phdr_num);
-        shdr = &shdr32;
-    } else {
-        shdr_size = sizeof(Elf64_Shdr);
-        memset(&shdr64, 0, shdr_size);
-        shdr64.sh_info = cpu_to_dump32(s, s->phdr_num);
-        shdr = &shdr64;
+    prepare_elf_section_hdrs(s);
+
+    ret = fd_write_vmcore(s->elf_section_hdrs, s->shdr_num * sizeof_shdr, s);
+    if (ret < 0) {
+        error_setg_errno(errp, -ret, "dump: failed to write section headers");
     }
 
-    ret = fd_write_vmcore(shdr, shdr_size, s);
-    if (ret < 0) {
-        error_setg_errno(errp, -ret,
-                         "dump: failed to write section header table");
-    }
+    g_free(s->elf_section_hdrs);
 }
 
 static void write_data(DumpState *s, void *buf, int length, Error **errp)
@@ -591,12 +620,10 @@ static void dump_begin(DumpState *s, Error **errp)
         return;
     }
 
-    /* write section to vmcore */
-    if (s->shdr_num) {
-        write_elf_section(s, 1, errp);
-        if (*errp) {
-            return;
-        }
+    /* write section headers to vmcore */
+    write_elf_section_headers(s, errp);
+    if (*errp) {
+        return;
     }
 
     /* write notes to vmcore */

include/sysemu/dump.h

@@ -177,6 +177,8 @@ typedef struct DumpState {
     int64_t filter_area_begin;  /* Start address of partial guest memory area */
     int64_t filter_area_length; /* Length of partial guest memory area */
 
+    void *elf_section_hdrs;     /* Pointer to section header buffer */
+
     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 */