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authorCarlos O'Donell <carlos@redhat.com>2013-05-14 00:06:35 -0400
committerCarlos O'Donell <carlos@redhat.com>2013-05-14 00:06:35 -0400
commit141af660d825d4443cbc5c24d29d57d6f8b0950f (patch)
tree5fbe6797772008e388849a6384384492e91e247f /elf/ldconfig.c
parent5d5ef5dbfc5be7aec31e5d33d28b2e93dc4b8a8d (diff)
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Add comments to vDSO hwcap loading process.
Loading of the vDSO pseudo-hwcap from the type 2 GNU note is a rather arcane and poorly documented process. Given that I had a chance to review this code today I thought I would add all of the things I had to lookup to verify the validity of the process. With a single .note.GNU the vDSO can register up to 64 flags, though in practice you are limited to 64 - _DL_FIRST_EXTRA bits which on x86 is 12 bits. The only use of this that I know of is in the Xen support in Linux where they use the 1st bit to indicate "nosegneg". I see "We use bit 1 to avoid bugs in some versions of glibc when bit 0 is used; the choice is otherwise arbitrary.", but no reference to a glibc bug anywhere. The code as-is should support bit zero, so we still have that free for future use. The kernel, glibc, and ld.so.cache must coordinate to ensure that bit values don't go too high and are used consistently. --- 2013-05-13 Carlos O'Donell <carlos@redhat.com> * elf/dl-hwcaps.c (_dl_important_hwcaps): Comment vDSO hwcap loading. * elf/ldconfig.c (is_hwcap_platform): Comment each hwcap check. (main): Comment "tls" pseudo-hwcap.
Diffstat (limited to 'elf/ldconfig.c')
-rw-r--r--elf/ldconfig.c6
1 files changed, 6 insertions, 0 deletions
diff --git a/elf/ldconfig.c b/elf/ldconfig.c
index 57c6a6f..340c132 100644
--- a/elf/ldconfig.c
+++ b/elf/ldconfig.c
@@ -173,13 +173,17 @@ is_hwcap_platform (const char *name)
{
int hwcap_idx = _dl_string_hwcap (name);
+ /* Is this a normal hwcap for the machine e.g. fpu? */
if (hwcap_idx != -1 && ((1 << hwcap_idx) & hwcap_mask))
return 1;
+ /* ... Or is it a platform pseudo-hwcap e.g. i686? */
hwcap_idx = _dl_string_platform (name);
if (hwcap_idx != -1)
return 1;
+ /* ... Or is this one of the extra pseudo-hwcaps that we map beyond
+ _DL_FIRST_EXTRA e.g. tls, or nosegneg? */
for (hwcap_idx = _DL_FIRST_EXTRA; hwcap_idx < 64; ++hwcap_idx)
if (hwcap_extra[hwcap_idx - _DL_FIRST_EXTRA] != NULL
&& !strcmp (name, hwcap_extra[hwcap_idx - _DL_FIRST_EXTRA]))
@@ -1265,6 +1269,8 @@ main (int argc, char **argv)
add_dir (argv[i]);
}
+ /* The last entry in hwcap_extra is reserved for the "tls"
+ pseudo-hwcap which indicates support for TLS. */
hwcap_extra[63 - _DL_FIRST_EXTRA] = "tls";
set_hwcap ();