From: "H.J. Lu" <hjl.tools@gmail.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: GNU C Library <libc-alpha@sourceware.org>
Subject: Re: [PATCH 2/3] elf: Set l_contiguous to 1 for the main map in more cases
Date: Sat, 15 Jan 2022 16:10:17 -0800 [thread overview]
Message-ID: <CAMe9rOrhEA9zKuG9YoQe7Qxbk18ybZ++vfZAb-XhSneMFTFanQ@mail.gmail.com> (raw)
In-Reply-To: <1c4f55eb03365f9ea7238e7ab4a997802081bf6e.1641228666.git.fweimer@redhat.com>
On Mon, Jan 3, 2022 at 9:12 AM Florian Weimer via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> l_contiguous was not initialized at all for the main map and
> always 0. This commit adds code to check if the LOAD segments
> are adjacent to each other, and sets l_contiguous accordingly.
> This helps _dl_find_object because it is more efficient if the
> main mapping is contiguous.
>
> Note that not all (PIE or non-PIE) binaries are contiguous in this
> way because BFD ld creates executables with LOAD holes:
>
> ELF LOAD segments creating holes in the process image on GNU/Linux
> https://sourceware.org/pipermail/binutils/2022-January/119082.html
> https://sourceware.org/bugzilla/show_bug.cgi?id=28743
> ---
> elf/rtld.c | 25 +++++++++++++++++++++++++
> 1 file changed, 25 insertions(+)
>
> diff --git a/elf/rtld.c b/elf/rtld.c
> index ba6e31377d..53293f5b13 100644
> --- a/elf/rtld.c
> +++ b/elf/rtld.c
> @@ -1144,6 +1144,22 @@ rtld_setup_main_map (struct link_map *main_map)
> main_map->l_map_start = ~0;
> /* And it was opened directly. */
> ++main_map->l_direct_opencount;
> + main_map->l_contiguous = 1;
> +
> + /* A PT_LOAD segment at an unexpected address will clear the
> + l_contiguous flag. The ELF specification says that PT_LOAD
> + segments need to be sorted in in increasing order, but perhaps
> + not all executables follow this requirement. Having l_contiguous
> + equal to 1 is just an optimization, so the code below does not
> + try to sort the segments in case they are unordered.
> +
> + There is one corner case in which l_contiguous is not set to 1,
> + but where it could be set: If a PIE (ET_DYN) binary is loaded by
> + glibc itself (not the kernel), it is always contiguous due to the
> + way the glibc loader works. However, the kernel loader may still
> + create holes in this case, and the code here still uses 0
> + conservatively for the glibc-loaded case, too. */
> + ElfW(Addr) expected_load_address = 0;
>
> /* Scan the program header table for the dynamic section. */
> for (const ElfW(Phdr) *ph = phdr; ph < &phdr[phnum]; ++ph)
> @@ -1207,12 +1223,21 @@ rtld_setup_main_map (struct link_map *main_map)
> if (main_map->l_map_start > mapstart)
> main_map->l_map_start = mapstart;
>
> + if (main_map->l_contiguous && expected_load_address != 0
> + && expected_load_address != mapstart)
> + main_map->l_contiguous = 0;
> +
> /* Also where it ends. */
> allocend = main_map->l_addr + ph->p_vaddr + ph->p_memsz;
> if (main_map->l_map_end < allocend)
> main_map->l_map_end = allocend;
> if ((ph->p_flags & PF_X) && allocend > main_map->l_text_end)
> main_map->l_text_end = allocend;
> +
> + /* The next expected address is the page following this load
> + segment. */
> + expected_load_address = ((allocend + GLRO(dl_pagesize) - 1)
> + & ~(GLRO(dl_pagesize) - 1));
> }
> break;
>
> --
> 2.33.1
>
LGTM.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
Thanks.
--
H.J.
next prev parent reply other threads:[~2022-01-16 0:10 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-03 17:11 [PATCH 0/3] Fix elf/tst-dl_find_objects with --enable-hardcoded-path-in-tests Florian Weimer
2022-01-03 17:11 ` [PATCH 1/3] elf: Introduce rtld_setup_main_map Florian Weimer
2022-01-16 0:11 ` H.J. Lu
2022-01-03 17:11 ` [PATCH 2/3] elf: Set l_contiguous to 1 for the main map in more cases Florian Weimer
2022-01-16 0:10 ` H.J. Lu [this message]
2022-01-03 17:11 ` [PATCH 3/3] elf/tst-dl_find_object: Disable subtests for non-contiguous maps (bug 28732) Florian Weimer
2022-01-14 15:06 ` H.J. Lu
2022-01-14 15:09 ` H.J. Lu
2022-01-14 15:10 ` Florian Weimer
2022-01-14 15:19 ` H.J. Lu
2022-01-14 15:39 ` Florian Weimer
2022-01-14 15:47 ` H.J. Lu
2022-01-14 15:51 ` Florian Weimer
2022-01-14 15:54 ` H.J. Lu
2022-01-14 15:56 ` Florian Weimer
2022-01-14 16:06 ` H.J. Lu
2022-01-14 16:12 ` Florian Weimer
2022-01-16 14:05 ` H.J. Lu
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