Align startOffset with p_align instead of pagesize for compatibility #510
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According to the ELF specification, the alignment of loadable process segments should satisfy (p_vaddr mod pagesize) == (p_offset mod pagesize). However, glibc earlier than 2.35 incorrectly requires that the LOAD segment be (p_vaddr mod p_align) == (p_offset mod p_align), and will output the error message "ELF load command address/offset not properly aligned" if this is not met. Since there are many systems that use glibc earlier than 2.35, it is preferable that newly added LOAD segments satisfy (p_vaddr mod p_align) == (p_offset mod p_align) for compatibility.
Since the p_align of the LOAD segment is no longer pagesize, the actual p_align value is used to calculate for the LOAD segment extension. If calculated with pagesize, new LOAD segment may be added even though the existing LOAD segment can be extended.
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According to the ELF specification, the alignment of loadable process segments should satisfy
(p_vaddr mod pagesize) == (p_offset mod pagesize).By 9f1c0af,
p_alignof the LOAD segment newly added by patchelf is no longer a fixed value pagesize. Then patchelf calculatesp_offsetto satisfy(p_vaddr mod pagesize) == (p_offset mod pagesize)according to the ELF specification. (p_offsetis rounded up by pagesize.)However, glibc earlier than 2.35 incorrectly requires that the LOAD segment be
(p_vaddr mod p_align) == (p_offset mod p_align). Since patchelf rounds upp_vaddrbyp_align,(p_vaddr mod p_align) == 0, but ifp_alignis not equal to pagesize,(p_offset mod p_align) == 0may not be true. glibc will output the following error message if the LOAD segment does not satisfy the alignment requirement.This is the commit when the issue is fixed in glibc.
https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=163f625cf9becbb82dfec63a29e566324129c0cd
Now, this is a regression caused by an older glibc, and the output of patchelf is correct in terms of the ELF specification. However, there are many systems that use glibc earlier than 2.35 (such as Ubuntu 20.04). For compatibility, it is preferable to calculate
p_offsetaccording to glibc requirements.Note that patchelf always places the section to be rewritten at the end of the file, as described in the comment at the top of
rewriteSectionsLibrary(). This can be a problem whenp_alignis large (For example, gcc on Ubuntu 18.04 seems to output a binary withp_align=0x200000) because the output file size expands per unit ofp_aligneach timerewriteSectionsLibrary()is called.Although, considering the restrictions of glibc, I think the code intended by 9f1c0af would be like this. To solve the problem of file size growth, it will be necessary to change the concept of "for dynamic libraries, we just place the replacement sections at the end of the file". For example, if a section rewrite results in a LOAD segment where no section is placed, then the section to be rewritten could be placed over it.
This may resolve issue #492.