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Stop APE bare metal loader from reading beyond program image end (#574)

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This allows e.g. `qemu-system-x86_64 -s o/examples/hello.com
-serial stdio` to work without having to add extra padding to
the end of the `hello.com` "disk image".

(The sector count computation is divided among two instructions
in the assembly code.  This is done on purpose, to prevent an
ASCII 0x27 (single quote) byte from appearing in the bare
metal loader code, which will break the shell script loader.
There is probably a better way.)

Co-authored-by: tkchia <tkchia-cosmo@gmx.com>
This commit is contained in:
tkchia 2022-09-04 15:19:08 +08:00 • committed by GitHub
parent 9c017c98d3
commit 8569704c1d
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2
ape/ape.S
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@ -250,6 +250,7 @@ pc: cld
xor %cx,%cx # current cylinder xor %cx,%cx # current cylinder
xor %dh,%dh # current head xor %dh,%dh # current head
mov $v_ape_realsectors,%di # total sectors mov $v_ape_realsectors,%di # total sectors
sub $v_ape_realslacksectors,%di
3: call pcread 3: call pcread
dec %di dec %di
jnz 3b jnz 3b
@ -1567,6 +1568,7 @@ kernel: movabs $ape_stack_vaddr,%rsp
.ldsvar _end .ldsvar _end
.ldsvar _etext .ldsvar _etext
.ldsvar v_ape_realsectors .ldsvar v_ape_realsectors
.ldsvar v_ape_realslacksectors
.ldsvar v_ape_highsectors .ldsvar v_ape_highsectors
.ldsvar ape_idata_ro .ldsvar ape_idata_ro
.ldsvar ape_pad_rodata .ldsvar ape_pad_rodata

3
ape/ape.lds
View file

@ -563,6 +563,9 @@ SHSTUB2(ape_loader_dd_count,
HIDDEN(v_ape_realsectors = HIDDEN(v_ape_realsectors =
MIN(0x70000 - IMAGE_BASE_REAL, MIN(0x70000 - IMAGE_BASE_REAL,
ROUNDUP(RVA(_edata), 4096)) / 512); ROUNDUP(RVA(_edata), 4096)) / 512);
HIDDEN(v_ape_realslacksectors =
v_ape_realsectors - MIN(0x70000 - IMAGE_BASE_REAL,
ROUNDUP(RVA(_edata), 512)) / 512);
HIDDEN(v_ape_realpages = v_ape_realsectors / (4096 / 512)); HIDDEN(v_ape_realpages = v_ape_realsectors / (4096 / 512));
HIDDEN(v_ape_highsectors = HIDDEN(v_ape_highsectors =
(ROUNDUP(RVA(_edata), 512) / 512) - v_ape_realsectors); (ROUNDUP(RVA(_edata), 512) / 512) - v_ape_realsectors);