loader: add API to load elf header

Add an API to load an elf header header from a file. Populates a
buffer with the header contents, as well as a boolean for whether the
elf is 64b or not. Both arguments are optional.

Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: Fix typo in comment]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/core/loader.c

@@ -354,6 +354,61 @@ const char *load_elf_strerror(int error)
     }
 }
 
+void load_elf_hdr(const char *filename, void *hdr, bool *is64, Error **errp)
+{
+    int fd;
+    uint8_t e_ident_local[EI_NIDENT];
+    uint8_t *e_ident;
+    size_t hdr_size, off;
+    bool is64l;
+
+    if (!hdr) {
+        hdr = e_ident_local;
+    }
+    e_ident = hdr;
+
+    fd = open(filename, O_RDONLY | O_BINARY);
+    if (fd < 0) {
+        error_setg_errno(errp, errno, "Failed to open file: %s", filename);
+        return;
+    }
+    if (read(fd, hdr, EI_NIDENT) != EI_NIDENT) {
+        error_setg_errno(errp, errno, "Failed to read file: %s", filename);
+        goto fail;
+    }
+    if (e_ident[0] != ELFMAG0 ||
+        e_ident[1] != ELFMAG1 ||
+        e_ident[2] != ELFMAG2 ||
+        e_ident[3] != ELFMAG3) {
+        error_setg(errp, "Bad ELF magic");
+        goto fail;
+    }
+
+    is64l = e_ident[EI_CLASS] == ELFCLASS64;
+    hdr_size = is64l ? sizeof(Elf64_Ehdr) : sizeof(Elf32_Ehdr);
+    if (is64) {
+        *is64 = is64l;
+    }
+
+    off = EI_NIDENT;
+    while (hdr != e_ident_local && off < hdr_size) {
+        size_t br = read(fd, hdr + off, hdr_size - off);
+        switch (br) {
+        case 0:
+            error_setg(errp, "File too short: %s", filename);
+            goto fail;
+        case -1:
+            error_setg_errno(errp, errno, "Failed to read file: %s",
+                             filename);
+            goto fail;
+        }
+        off += br;
+    }
+
+fail:
+    close(fd);
+}
+
 /* return < 0 if error, otherwise the number of bytes loaded in memory */
 int load_elf(const char *filename, uint64_t (*translate_fn)(void *, uint64_t),
              void *translate_opaque, uint64_t *pentry, uint64_t *lowaddr,

include/hw/loader.h

@@ -48,6 +48,19 @@ int load_elf(const char *filename, uint64_t (*translate_fn)(void *, uint64_t),
              void *translate_opaque, uint64_t *pentry, uint64_t *lowaddr,
              uint64_t *highaddr, int big_endian, int elf_machine,
              int clear_lsb);
+
+/** load_elf_hdr:
+ * @filename: Path of ELF file
+ * @hdr: Buffer to populate with header data. Header data will not be
+ * filled if set to NULL.
+ * @is64: Set to true if the ELF is 64bit. Ignored if set to NULL
+ * @errp: Populated with an error in failure cases
+ *
+ * Inspect an ELF file's header. Read its full header contents into a
+ * buffer and/or determine if the ELF is 64bit.
+ */
+void load_elf_hdr(const char *filename, void *hdr, bool *is64, Error **errp);
+
 int load_aout(const char *filename, hwaddr addr, int max_sz,
               int bswap_needed, hwaddr target_page_size);
 int load_uimage(const char *filename, hwaddr *ep,