Re: [PATCH dwarves v3 2/5] btf_encoder: Do not use both structs and pointers for the same data

[Giuliano Procida <gprocida@google.com>]

Hi.

On Mon, 8 Feb 2021 at 22:23, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>
> On Fri, Feb 5, 2021 at 5:42 AM Giuliano Procida <gprocida@google.com> wrote:
> >
> > Many operations in the libelf API return a pointer to a user-provided
> > struct (on success) or NULL (on failure).
> >
> > There are a couple of places in btf_elf__write where both structs and
> > pointers to the same structs are used. Holding on to the pointers
> > raises ownership and lifetime issues unncessarily and the code is
>
> typo: unnecessarily
>

Thanks. Fixed.

> > cleaner with only a single access path for these data.
> >
> > The code now treats the returned pointers as booleans.
> >
> > Signed-off-by: Giuliano Procida <gprocida@google.com>
> > ---
>
> styling nits, but otherwise LGTM
>
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
>
> >  libbtf.c | 14 ++++++--------
> >  1 file changed, 6 insertions(+), 8 deletions(-)
> >
> > diff --git a/libbtf.c b/libbtf.c
> > index 7bc49ba..ace8896 100644
> > --- a/libbtf.c
> > +++ b/libbtf.c
> > @@ -698,8 +698,7 @@ int32_t btf_elf__add_datasec_type(struct btf_elf *btfe, const char *section_name
> >
> >  static int btf_elf__write(const char *filename, struct btf *btf)
> >  {
> > -       GElf_Shdr shdr_mem, *shdr;
> > -       GElf_Ehdr ehdr_mem, *ehdr;
> > +       GElf_Ehdr ehdr;
> >         Elf_Data *btf_data = NULL;
> >         Elf_Scn *scn = NULL;
> >         Elf *elf = NULL;
> > @@ -727,13 +726,12 @@ static int btf_elf__write(const char *filename, struct btf *btf)
> >
> >         elf_flagelf(elf, ELF_C_SET, ELF_F_DIRTY);
> >
> > -       ehdr = gelf_getehdr(elf, &ehdr_mem);
> > -       if (ehdr == NULL) {
> > +       if (!gelf_getehdr(elf, &ehdr)) {
> >                 elf_error("elf_getehdr failed");
> >                 goto out;
> >         }
> >
> > -       switch (ehdr_mem.e_ident[EI_DATA]) {
> > +       switch (ehdr.e_ident[EI_DATA]) {
> >         case ELFDATA2LSB:
> >                 btf__set_endianness(btf, BTF_LITTLE_ENDIAN);
> >                 break;
> > @@ -751,10 +749,10 @@ static int btf_elf__write(const char *filename, struct btf *btf)
> >
> >         elf_getshdrstrndx(elf, &strndx);
> >         while ((scn = elf_nextscn(elf, scn)) != NULL) {
> > -               shdr = gelf_getshdr(scn, &shdr_mem);
> > -               if (shdr == NULL)
> > +               GElf_Shdr shdr;
>
> it's a good style to have an empty line between variable declaration
> block and subsequent instructions
>

The variable in this question is effectively initialised by the
statement on the next line, breaking them apart looks odd.
Also, this is not a variable that needs end-of-scope clean-up. Its
position at the top of the scope is coincidental.
Later commits in the series also place declaration and initialisation
as close together as possible.
The only variables I would intentionally place at the top of a given
scope *and* far from their natural points of initialisation are those
corresponding to resources that need to be released at the end of the
scope, with the labelled exit idiom.
I feel this gives a better balance between readability (keeping things
local) and keeping track of resources (memory, fds, other handles) in
a scope.

However, if that's contrary to the house style, it's easy enough to
pull all the declarations out and move them to the top and separate
them; the compiler should be clever enough to share stack slots in any
case.

Let me know.

Regards,
Giuliano.

>
> > +               if (!gelf_getshdr(scn, &shdr))
> >                         continue;
> > -               char *secname = elf_strptr(elf, strndx, shdr->sh_name);
> > +               char *secname = elf_strptr(elf, strndx, shdr.sh_name);
> >                 if (strcmp(secname, ".BTF") == 0) {
> >                         btf_data = elf_getdata(scn, btf_data);
> >                         break;
> > --
> > 2.30.0.478.g8a0d178c01-goog
> >