Hi Aaron,
On Mon, 2025-08-04 at 23:24 -0400, Aaron Merey wrote:
> Signed-off-by: Aaron Merey <ame...@redhat.com>
> ---
> v2: Remove mention of implementation details. Mention that conversion
> to native byte order and direct access alignment resembles elf_getdata.
> Mention elf_errmsg.
>
> On Mon, Jul 7, 2025 at 1:26 PM Mark Wielaard <m...@klomp.org> wrote:
> >
> > Nice overview of ELF data types. But might there be a better place for
> > this? I would assume elf_data, but that man page is currently very bare
> > bones.
>
> When I update the bare bones man pages including elf_data I'll move
> the Elf_Type description there.
Thanks.
> > Speaking of thread safety...
> > This should be MT-Safe and documented that way.
> >
> > But is it really?
> > The code takes a read lock on elf->lock. Checks whether it can find an
> > existing Elf_Data_Chunk. If yes, it returns that one. Which seems fine.
> > But if not it inserts the dummy chunk without actually data,
> > allocates/creates the data, drops the rdlock (!), takes a write lock
> > and overwrites the dummy chunk with the real data, drops the lock again
> > and returns the data.
> >
> > What if at (!) another call gets the read lock first before the current
> > thread can (re)take the write lock? That other thread could find the
> > existing dummy key already in the cache and return the dummy data?
>
> I've posted a patch that fixes this race condition.
I cannot find it, could you repost it please?
> doc/Makefile.am | 1 +
> doc/elf_getdata_rawchunk.3 | 140 +++++++++++++++++++++++++++++++++++++
> 2 files changed, 141 insertions(+)
> create mode 100644 doc/elf_getdata_rawchunk.3
>
> diff --git a/doc/Makefile.am b/doc/Makefile.am
> index 791a68da..fbe69980 100644
> --- a/doc/Makefile.am
> +++ b/doc/Makefile.am
> @@ -67,6 +67,7 @@ notrans_dist_man3_MANS= elf32_checksum.3 \
> elf_getarsym.3 \
> elf_getbase.3 \
> elf_getdata.3 \
> + elf_getdata_rawchunk.3 \
> elf_getscn.3 \
> elf_hash.3 \
> elf_kind.3 \
OK.
> diff --git a/doc/elf_getdata_rawchunk.3 b/doc/elf_getdata_rawchunk.3
> new file mode 100644
> index 00000000..9660b1f1
> --- /dev/null
> +++ b/doc/elf_getdata_rawchunk.3
> @@ -0,0 +1,140 @@
> +.TH ELF_GETDATA_RAWCHUNK 3 2025-06-30 "Libelf" "Libelf Programmer's Manual"
> +
> +.SH NAME
> +elf_getdata_rawchunk \- create a raw data descriptor from a file offset
> +
> +.SH SYNOPSIS
> +.nf
> +#include <libelf.h>
> +
> +.B Elf_Data * elf_getdata_rawchunk("Elf *elf", "int64_t offset", "size_t
> size", "Elf_Type type");
> +.fi
OK.
> +.SH DESCRIPTION
> +The
> +.BR elf_getdata_rawchunk ()
> +function returns an
> +.B Elf_Data
> +descriptor that refers to a raw region of the ELF file, starting at the
> +given file
> +.I offset
> +and spanning
> +.I size
> +bytes. This is used to access arbitrary byte ranges in the ELF file that may
> +not correspond to any section or segment.
> +
> +The returned
> +.B Elf_Data
> +.I d_buf
> +will be properly aligned for the requested
> +.IR type .
> +
> +If the file’s byte order matches the host, and alignment permits, the raw
> data is
> +returned as-is. If the file’s byte order differs, the buffer is converted
> into native
> +byte order and aligned for direct access. This behaves like
> +.BR elf_getdata (),
> +and not
> +.BR elf_rawdata ().
> +The contents of the return value's
> +.I d_buf
> +are a decoded representation.
> +
> +The returned data is not associated with any section or update mechanism. It
> will not
> +be included in any output written by
> +.BR elf_update (3).
> +It is also owned by libelf and is valid until
> +.BR elf_end ()
> +is called on
> +.IR elf .
OK.
> +.SH PARAMETERS
> +.TP
> +.I elf
> +A pointer to an
> +.B Elf
> +descriptor representing the ELF file.
> +
> +.TP
> +.I offset
> +The starting file offset in bytes. This must lie within the bounds of the
> file.
> +
> +.TP
> +.I size
> +The number of bytes to include in the data chunk, starting at
> +.IR offset .
> +
> +.TP
> +.I type
> +An
> +.B Elf_Type
> +enumeration value indicating the intended interpretation of the data.
> Values include:
> +
> +.RS
> +.TP
> +.B ELF_T_BYTE
> +Raw bytes.
> +.TP
> +.B ELF_T_ADDR, ELF_T_OFF, ELF_T_HALF, ELF_T_WORD, ELF_T_XWORD, ELF_T_SWORD,
> ELF_T_SXWORD
> +Integer and pointer types.
> +.TP
> +.B ELF_T_EHDR, ELF_T_SHDR, ELF_T_PHDR
> +ELF file, section, and program headers.
> +.TP
> +.B ELF_T_SYM, ELF_T_DYN, ELF_T_REL, ELF_T_RELA, ELF_T_RELR
> +Symbols and relocations.
> +.TP
> +.B ELF_T_NHDR, ELF_T_CHDR, ELF_T_NHDR8
> +ELF note headers and compressed data headers.
> +.TP
> +.B ELF_T_VDEF, ELF_T_VDAUX, ELF_T_VNEED, ELF_T_VNAUX
> +Versioning structures.
> +.TP
> +.B ELF_T_SYMINFO, ELF_T_MOVE, ELF_T_LIB
> +Other ELF metadata.
> +.TP
> +.B ELF_T_GNUHASH
> +GNU-style hash table.
> +.TP
> +.B ELF_T_AUXV
> +Auxiliary vectors.
> +.RE
OK.
> +.SH RETURN VALUE
> +Returns a pointer to an
> +.B Elf_Data
> +descriptor on success.
> +
> +Returns NULL if any arguments are invalid, or if memory allocation or file
> reading
> +fails. An error code is set which can be retrieved with
> +.BR elf_errmsg ().
> +The result is cached and shared between repeated calls using the same
> arguments.
OK.
> +.SH SEE ALSO
> +.BR elf (3),
> +.BR elf_errmsg (3),
> +.BR elf_getdata (3),
> +.BR elf_getscn (3),
> +.BR elf_rawdata (3),
> +.BR libelf (3),
> +.BR elf (5)
OK.
> +.SH ATTRIBUTES
> +.TS
> +allbox;
> +lbx lb lb
> +l l l.
> +Interface Attribute Value
> +T{
> +.na
> +.nh
> +.BR elf_getdata_rawchunk ()
> +T} Thread safety MT-Safe
> +.TE
> +
> +.SH REPORTING BUGS
> +Report bugs to <elfutils-devel@sourceware.org> or
> https://sourceware.org/bugzilla/.
> +
> +.SH HISTORY
> +.B elf_getdata_rawchunk
> +first appeared in elfutils 0.130. This function is a elfutils libelf
> extension and
> +may not be available in other libelf implementations.
OK.
