Re: [Qemu-devel] [PATCH v1 15/17] loader: add API to load elf header

[Peter Crosthwaite]

On Tue, Jan 19, 2016 at 9:50 AM, Peter Maydell <address@hidden> wrote:
> On 18 January 2016 at 07:12, Peter Crosthwaite
> <address@hidden> wrote:
>> 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 <address@hidden>
>> ---
>>
>>  hw/core/loader.c    | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
>>  include/hw/loader.h |  1 +
>>  2 files changed, 49 insertions(+)
>>
>> diff --git a/hw/core/loader.c b/hw/core/loader.c
>> index 6b69852..28da8e2 100644
>> --- a/hw/core/loader.c
>> +++ b/hw/core/loader.c
>> @@ -331,6 +331,54 @@ 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[EI_NIDENT];
>> +    size_t hdr_size, off = 0;
>> +    bool is64l;
>> +
>> +    fd = open(filename, O_RDONLY | O_BINARY);
>> +    if (fd < 0) {
>> +        error_setg_errno(errp, errno, "Fail to open file");
>
> "Failed" (also below).
>

Fixed (x2).

> I don't think we end up with the filename anywhere in the
> error message; it would be helpful if we could include it.
>

Fixed (x4)

>> +        return;
>> +    }
>> +    if (read(fd, e_ident, sizeof(e_ident)) != sizeof(e_ident)) {
>> +        error_setg_errno(errp, errno, "Fail to read file");
>> +        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;
>> +    }
>> +
>> +    lseek(fd, 0, SEEK_SET);
>
> You're not checking this lseek for failure (and you don't
> need it anyway, because you could just copy the magic bytes
> into *hdr and read four fewer bytes).
>

OK, so I have optimised it away. What I am doing now is always reading
to straight to hdr[], and if the caller passes hdr == NULL, then hdr
is set to a local buffer (and the full header read is still skipped as
per current logic).

>> +    while (hdr && off < hdr_size) {
>> +        size_t br = read(fd, hdr + off, hdr_size - off);
>> +        switch (br) {
>> +        case 0:
>> +            error_setg(errp, "File too short");
>> +            goto fail;
>> +        case -1:
>> +            error_setg_errno(errp, errno, "Failed to read file");
>> +            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,
>> diff --git a/include/hw/loader.h b/include/hw/loader.h
>> index f7b43ab..33067f8 100644
>> --- a/include/hw/loader.h
>> +++ b/include/hw/loader.h
>> @@ -36,6 +36,7 @@ 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);
>> +void load_elf_hdr(const char *filename, void *hdr, bool *is64, Error 
>> **errp);
>
> Doc comment, please.
>

Added:

+
+/** 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 as ELF file's header. Read its full header contents into a
+ * buffer and/or determine if the ELF is 64bit.
+ */


Regards,
Peter

>>  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,
>> --
>> 1.9.1
>
> thanks
> -- PMM