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ld.so.8: Tweaks to H.J. Lu's patch 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2015-12-16 14:59:39 +01:00
parent 5b479f37b6
commit 3cdc1fc4a1
1 changed files with 14 additions and 5 deletions
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19
man8/ld.so.8
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@ -474,13 +474,22 @@ If set to a nonempty string, warn about unresolved symbols.
.TP .TP
.B LD_PREFER_MAP_32BIT_EXEC .B LD_PREFER_MAP_32BIT_EXEC
(x86-64 only)(glibc since 2.23) (x86-64 only)(glibc since 2.23)
According to Silvermont software optimization guide, for 64-bit According to the Intel Silvermont software optimization guide, for 64-bit
applications, branch prediction performance can be negatively impacted applications, branch prediction performance can be negatively impacted
when the target of a branch is more than 4GB away from the branch. when the target of a branch is more than 4GB away from the branch.
If set, ld.so will try to map executable pages with MAP_32BIT first. If this environment variable is set (to any value),
NB: MAP_32BIT will map to lower 2GB, not lower 4GB, address. Since .BR ld.so
MAP_32BIT reduces bits available for address space layout randomization will first try to map executable pages using the
(ASLR), LD_PREFER_MAP_32BIT_EXEC is always disabled for SUID programs. .BR mmap (2)
.BR MAP_32BIT
flag, and fall back to mapping without that flag if that attempt fails.
NB: MAP_32BIT will map to the low 2GB (not 2GB) of the address space.
Because
.B MAP_32BIT
reduces the address range available for address space layout
randomization (ASLR),
.B LD_PREFER_MAP_32BIT_EXEC
is always disabled in secure-execution mode.
.TP .TP
.B LDD_ARGV0 .B LDD_ARGV0
(libc5) (libc5)